Showing posts with label 9 year old. Show all posts

Monday, March 6, 2017

Cryptarithmetic puzzles follow-up

I was asked to write a bit about strategies and answers for the puzzles we gave two weeks ago.

BIG + PIG = YUM
Because the digits in YUM are all distinct from BIG and PIG and there are only 7 letters in this puzzle, we should expect there to be many solutions.

The easiest way to get a feel for the puzzle is to start trying values and see what develops. This was part of the idea of using this puzzle as the opening challenge.

As we play with examples, the kids should notice these things that constrain our possible solutions:

B, G, I, M, P, U, Y must all be distinct
We are adding two three digit numbers and the sum is a three digit number
B, P, and Y are all leading digits
The largest sum possible with two numbers 0 to 9 is 18.

Some conclusions:

(a) G is not 0. If it was, then M would also be 0.

(b) B, P, and Y are all not 0. They are leading digits, the rules of our puzzles say they can't be zero.

(c) G + G is at most 18. It may contribute at most one ten to the calculation of U. That will only happen if G is 5 or larger.

(d) I + I is at most 18. Along with a potential ten from G+G, that means we have at most 19 coming from the tens. That will only happen if I is 5 or larger.

(e) B+G is at most 9. If there is an extra hundred coming from the tens digits, B+ G is at most 8.

(f) If I is 9, G must be less than 5. Can you see why?

(g) If G is less than 5, I cannot be 0

After these observations, I'd suggest picking values of G, then seeing what values of I are allowed, then checking what remains for B and P. Because we aren't allowed to have duplicates, we quickly see that our choices are constrained.

For example, if G is 1 or 2, then I is at least 3 and we get the following possible solutions (B and P can be interchanged):

431 + 531 = 962

341 + 641 = 982

351 + 451 = 802

371 + 571 = 942

381 + 581 = 962

132 + 732 = 864

132 + 832 = 964

152 + 652 = 804

152 + 752 = 904

182 + 582 = 764

192 + 392 = 584

192 + 592 = 784

There are some more advanced ideas that could come out of trying to count or list all of the solutions, so I'd encourage people to explore. Even this simple puzzle can be a lot of fun!

CAT + HAT = BAD

The A in BAD is the key part of this puzzle. We can get two cases:

(a) A is 0 and T is 1, 2, 3 or 4

(b) A is 9 and T is 5, 6, 7 or 8.

Again, while there are a lot of solutions (and counting them would be a fun challenge) they are easiest to build up by choosing A (either 0 or 9), then T, then seeing what flexibility is left for C and H. Here are some examples:

301 + 401 = 702

301 + 501 = 802

301 + 601 = 902

302 + 502 = 804

302 + 602 = 904

103 + 403 = 506

395 + 495 = 890

SAD + MAD + DAD = SORRY

This was a puzzle without a solution. In this case, it isn't too hard to see that SORRY has too many digits. The best explanation was given by one student:

The largest three digit number is 999.
If we add three of them, we will at most get 2997.
SORRY has to be bigger than 10,000.
This isn't possible

CURRY + RICE = LUNCH

Unfortunately, this also doesn't have a solution, but the reasoning is more subtle than the previous puzzle.

Here, we can reason as follows:

R cannot be 0 because it is the leading digit in RICE
Because the tens digit of RICE and LUNCH are both C, R must be 9 and we must have Y + E > 10.
This also means R + C + 1 = 10 + C.
That will mean the 100s digit of RICE must be the same as the 100s digit of the sum.
However, the 100s digit of RICE and LUNCH are different.

Too bad, it was such a cute puzzle!

ALAS + LASS + NO + MORE = CASH

This is the most challenging puzzle from this set.

Some things we notice:

There are ten letters (A C E H L M N O R S) and they must all be distinct.
We are adding three 4-digit numbers and a two digit number to produce another 4 digit number.
A, L, N, M and C are leading digits, so they can't be zeros.
The tens and hundreds digits of CASH (S and A) are also involved in the sums for those digits.

Point 4 has a subtle implication, which I'll illustrate with the hundreds digits. Since L + O must be more than 0, but A is the hundreds digit of the sum, we must have some number of thousands carried over. Because A, L and M are all distinct and larger than 0, the smallest their sum can be is 1+2+3. Putting these two observations together, C must be at least 7.

In this case, I find it helpful to put together a table showing possibilities that we have eliminated:

We can see some more restrictions from the fact that A + L + M must be less than 9. That means we have only the following possible triplets (ignoring order):

{1, 2, 3}, {1, 2, 4}, {1, 2, 5}, {1, 3, 4}

One thing we notice is that 1 is in all of these triplets, so either A, L or M must be 1 and none of the other letters can be 1. Another thing we notice is that we don't yet have any way of differentiating A, L, or M, so any ordering of our triplets is possible. That would mean we have 24 cases to consider.

Let's see how we would work through the cases, starting with A = 1, L = 2, M = 3, the first on our list. Now this, happens to be a stroke of luck, as we'll see.

Starting from the thousands digit, we see that this would make C = 7, if there is a single carry from the hundreds. Indeed, we can see that this must be the value (in the case we are testing), as the carry from there could only come from L + O (plus any carry from the tens digit). Since L is at most 5, L + O is at most 14 and any carry from the tens digit must be less than 6.

Now, in the hundreds digit, we have 2 + O + carry from the tens = 10, so O = 8 - carry from tens.
We know there must be at least one carry from the tens, so O is at most 7. Since 7 is already used by C, let's try 6. That means we need to get 2 hundreds carried over from the tens, so we need
A + N + R + carry from ones = 20, or N + R + carry from ones = 19. Since we have already used 6 and 7, the only way this is possible is if N and R are 8 and 9 (in either order) and we are carrying 2 from the ones.

At this point, the case we've worked through has:
121S + 21SS + 86 + 369E = 71SH

We still have to allocate digits 0, 4, and 5. and we know that S + S + 6 + E = 20 + H. Given our remaining digits, the biggest the left hand can be is if S is 5 and E is 4, making 20. The smallest the right hand can be is if H is 0. Fortunately, this makes the equality hold, so we get our final answer:

1255 + 2155 + 86 + 3694 = 7150

Through the process of checking this case, we learned more about how the carry from lower digits is restricted and it would be faster for us to check through remaining cases.
Let me know how many other solutions you find!

LOL + LOL + LOL + LOL + LOL + LOL + LOL + LOL + LOL + LOL +

LOL + LOL + LOL + LOL + LOL + LOL + LOL + LOL + LOL + LOL + LOL = ROFL

There are 71 LOLs, so this is 71 x LOL = ROFL. While this looks daunting, there are some ideas which take us a long way to the solution.

First, ROFL has 4 digits. If L were 2, 71 x LOL would be more than 14,000, so L must be 1. In fact, ROFL is less than 9861, so LOL is smaller than 9871 / 71 which is 139. We can quickly check

101, 121, and 131 and see that 131 works.

71 x 131 = 9301

Thursday, February 23, 2017

A bit of 3D(ish) geometry

Note: this post started out focused on two recent geometry projects. However, the Desmos Function Carnival, which I originally just included as a miscellaneous item, is also worth your time.

Nets and solids

The most ambitious project recently was led by P (the mom, of course). She found nets for 3-d shapes and supervised J1 and J2 as they created a nice display to take to school.

Here are the nets: SenTeacher Polyhedral nets. They have a collection of other printables, but this collection of nets seems to be the most interesting. Take a look and let me know if you see anything else worthwhile.

Here's the completed board:

A profile picture to show that these are really 3-d:

Solid Shapes and Their Nets (I think Jo Edkins is the author) has a nice discussion of nets and a little puzzle game to distinguish nets that fold into the platonic solids and which don't. Feel free to try to do this for the icosahedron or dodecahedron!

A two dimensional challenge?

J2 asked me about a triangle with two right angles. Of course, we all know that a triangle can't have two 90 degree angles, right? Well, this fits in my list of math lies from this old post: 23 isn't prime.

We talked briefly about triangles on a plane and agreed that two 90 degree angles wouldn't work. If we try by starting with a side and building two right angles on that side, we just get parallel lines. Ok, that's standard.

However, what about this:

We discussed spherical geometry for a bit. Not shown is our first attempt on the other side of the tennis ball that did have two right angles, but the third wasn't. He wanted to see an equiangular triangle on a sphere. This led to further discussion of life on a sphere:

what is the largest interior angle sum for a triangle?
what is the smallest interior angle sum?
are there any parallel lines?
are there any squares? are there even any rectangles?
is π (the ratio of the circumference to diameter of a circle) a constant?
do we have to think like this in real life because the earth is close to a sphere?

Picking up the point about π, it occurs to me that this is another math lie. This point got a nice treatment recently by SMBC:

I made this pic small so you will go to the site and look at Zach Weinersmith's other awesome work

Some open follow-ups:

what is an equiangular quadrilateral on the sphere?
what about hyperbolic geometry? I think no triangles with two right angles. If I recall correctly, triangles there have angle sums smaller than 180 degrees (or π radians, ha!)

Other Misc Math

Now for my usual grab bag of other things we've been doing. Some of these are really great activities, so don't skimp on this section!

Desmos Function Carnival
We learned about the Desmos Function Carnival activity from a post by Kent Haines which, in turn, we learned about via Michael Pershan's post. They both were writing about teaching functions to their students and have outlined a really nice sequencing of lessons, if you're into that kind of thing.

In case you are, you might like to know that there is another flavor of Function Carnival available through the Desmos teacher site with 2 other activities. Each version is worth checking out because both of worthwhile activities that aren't in the other. Also, Kent links to this nice graphing activity from the Shell Center which also worked well with J1 and J2.

For our purposes, I was more interested in the mathematical physics (plots of position vs time or velocity vs time) and the fun of creating wacky animations with impossible plots.

If I can figure out how to post the animations, I will update this page, since the animations really enhance the experience. For now, let me give you some screen caps of some of their proposed graphs from the Cannon Man (height) activity:

Have a free-form drawing tool? Make your name!

Note: the vertical stripe doesn't really work with this animation, but the patriotic spirit is there!

Our first attempt at Quantum Field Theory

Here is J2 playing with the function carnival, but J3 (4 years old) enjoyed it just as much:

Different representations
J3 is working on place value and playing with different representations of numbers. Here, she's got the 100 board, an abacus, and foam decimal models. Sometimes I challenge her to make a number, sometimes she challenges me.

Tuesday, February 21, 2017

RSM International Math Contest

Sometime the first week of February, the RSM offers an on-line math competition. For the second year, I had J1 and J2 work through the problems at grade 3 and grade 4 level. This post is about our thoughts on competition, but I end with a problem from the contest that had us debating.

Compete and win!

To get it out of the way, let me explain my interest in the competition and why I had the kids enter. Above all, I was curious about the problems and expected they would be interesting challenges. We found the puzzles last year interesting, so I was pretty sure this year's collection would also be nice.
While I know that there are tons of excellent activities, I couldn't resist making use of this resource when someone thoughtful had put together a convenient collection in one place.

Second, I am curious about levels and assessments. What types of questions does RSM think 3rd and 4th graders should be able to answer, but find challenging? How difficult would our two little ones find them? Since we are currently operating outside a standard curriculum framework, it is hard for me to judge where they are or what we should expect of them. Admittedly, there are other ways I could form this assessment, but they take more effort.

Notice that I don't really care about how well they perform on the test. It is interesting information for me, but I don't need them to do well.

P has a different view. She was an olympiad kid and sees competitions as the easiest way for our kids to distinguish themselves. You know the anxiety: if they don't win competitions, they won't get into Harvard, they'll end up on the street somewhere.

Perhaps unsurprisingly, the kids are getting a mixed message about the importance and reason for participating in competitions. This year, J1 was particularly sensitive. He was very resistant to doing the RSM test. I spent a lot of time talking with him. Mainly, I wasn't hoping to convince him to do the test, but I wanted to explore other issues related to the expectations he feels on himself, his relationship with J2, and his mindset about his own learning.

I am trying to communicate:

effort and progress are important, starting point and base ability aren't. I know this is debatable, but I'm talking about the differences between my kids where I'm on pretty firm ground.
there are many aspects to math and mathematical ability. Calculating and answering questions quickly are facets, logical reasoning, strategic thinking, spatial reasoning, asking good questions, gathering data, exploring connections, etc, etc are all components, too.
time isn't important. One issue with the RSM test is that it is timed. This goes in the face of our repeated efforts to emphasize that the time it takes to solve something is not a key consideration. To help with this, I screen capped the questions so he could work on them after the official time had expired. Note, however, his recorded performance on the test was based on work within the contest rules.
Math is not about right answers.

The next section might help illustrate that last point.

Which area do we want?

As mentioned above, one of the questions from grade 4 cause us to argue among ourselves. I'm paraphrasing slightly:

Two tennis ball machines stand on opposite ends of a 25 meter by 10 meter court. The yellow machine shoots yellow balls that stop on the court 2 meters to 16 meters from the yellow machine's side. The green machine shoots green balls that stop on the court 5 meters to 20 meters from the green machine's side. Find the area of the court that has balls of either color on it.

Here are some ideas from our discussion:

Option A: we want to find the area of overlap, because that's the only place we could find balls of either color.
Option B: we want to find the combined areas, because that's where we could find tennis balls, either color.
Option 1: the area with green balls is a rectangle, the area with yellow balls is a rectangle.
Option 2: think of the ball machines as single points, shooting balls at various angles and various distances. the target area for each machine is an intersection of of a circular ring and a rectangle. This idea came from J2 when we were looking over the grade 4 questions after time had expired.

To answer the question, choose either A xor B and choose either 1 xor 2. So, what is the right answer?

I think the right answer is to have this discussion, to encourage multiple interpretations (with justification), to see how the answers compare, to think about what other things we could do to make the intended interpretation more clear (a diagram seems the most obvious), to recognize that this is part of the richness of math and it can't be represented by a single numerical answer on a timed test.

Cryptarithmetic Puzzles for Grades 1 to 4

Inspired by a series of puzzles from Manan Shah, I decided to have the kids play with cryptarithmetic puzzles today. In addition to borrowing some of Manan's puzzles, I also used some from this puzzle page: Brain Fun. I've included some more comments below about the Brain Fun puzzles.

My main concern was whether the puzzles were at the right level. In particular, I was afraid that the puzzles would be too hard. In fact, I tried solving a bunch of them yesterday and actually found myself struggling. I'll ascribe some of that to being tired and sick. However, my intuition was to make some simpler puzzles of my own. In particular, I added:

puzzles that have many solutions: I figured that many solutions would make it easy to find at least one.
a puzzle that "obviously" has no solution. Now, obviously, the word "obviously" is a sneaky one in math, but I was pretty sure the kids could see the problem with this structure.

Grades 1 and 2

For the younger kids, I started with a shape substitution puzzle. This is one our family explored almost 2 years ago: Shape Substitution. I don't recall the original source.

Two reasons why I started with this. First, it has a lot of solutions, but there is an important insight that unlocks those solutions. Second, by using shapes, we can write possible number solutions inside them as we solve or guess-and-check the puzzle. This made it easier for the kids to see the connection that all squares have the same value, etc.

The second puzzle: BIG + PIG = YUM

Really just a warm-up practicing the rules and doing a little bit of checking that we haven't duplicated any numbers.

The third puzzle: CAT + HAT = BAD

Again, lots of solutions, but noticing leads to a good insight.

Fourth puzzle: SAD + MAD + DAD = SORRY

This is a trick puzzle. The kids know that I like to tease them, so they are aware they need to look out for things like this. We discussed this in class and I suggested they give this puzzle to their parents.

Fifth puzzle: CURRY + RICE = LUNCH

When I translated this to Thai, all the kids laughed. I was sneaking a little bit of English practice into the lesson and then they realized that it was worth trying to read all the puzzles, not just solve them.

Sources: I think I made up all of these puzzles (original authors, please correct me if I'm wrong).

Grades 3 and 4

The older kids already had experience with these puzzles. We did refresh their memory a bit with BIG + PIG = YUM

I asked them to give me the rules and explain why those rules made sense. As with most games, I want to communicate that we're doing things for a reason, but those reasons can be challenged. If they think it makes sense to do it a particular way, we're open to their ideas.

Second puzzle: SAD + MAD + DAD = SORRY

Same discussion as for the younger kids. When prompted, this was pretty easy for them to spot, but they weren't naturally attuned to think about whether a puzzle had solutions or how many. This led me to take a vote on all the puzzles at the end to see who thought the puzzles would have 0, 1 or many solutions.

Third puzzle: ALAS + LASS + NO + MORE = CASH

A puzzle from Brain Fun. I think this is one of the easier ones on that page. Again, a bit of English practice.

Fourth puzzle: LOL + LOL + LOL + .... + LOL = ROFL (71 LOLs)

This was from Manan. I think it is one of the easier ones in his collection, but it looks daunting. Turns out none of the kids in the class were familiar with (English) texting short-hand, so my attempt to be cool fell flat.

Fifth puzzle: CURRY + RICE = LUNCH

Again, everyone was delighted when I translated this one. We're in Thailand, after all, so at least one puzzle had to be about food.

The key exercise

The final assignment everyone (all four grades) was given was to make up a puzzle for me to solve. I was thinking it would be nice to have one in Thai, but we decided to keep it in English as further language practice.

Manan wrote a nice post about having kids design their own puzzles. If it goes well, this is actually the activity that ties a lot of the learning messages together: they think about structure, they think about what allows multiple or single solutions, they apply their own aesthetic judgment, they use their knowledge of the operations, they are empowered with an open-ended task that cannot be "wrong."

We'll see how it goes. At the very least, I expect a lot of work for myself when their puzzles come in!

An extra sweetener

Two kids asked if we could use other operations than addition. That prompted me to put this on the table (also from Brain Fun):

DOS x DOS = CUATRO

Brain Fun Problems

The first time I'd seen the Brain Fun problems, I added them to a list and called them "basic" (see this page.) When I actually went to solve them, though, they didn't seem so easy.

Big confession time: I actually looked at some of the solutions. However, I was disturbed to see that the solutions involved extra information that wasn't included as part of the problem statement! For example, in THREE + THREE + FIVE = ELEVEN, the solution assumes that ELEVEN is divisible by 11. This seems to be the case for several of the puzzles involving written out arithmetic:

TWO + TWENTY = TWELVE + TEN (assume 20 divides TWENTY and 12 divides TWELVE, I wasn't clear about whether any divisibility was assumed for TWO and TEN)

I'm not sure if similar assumptions are allowed/required for any of the others.

Maybe I shouldn't complain, since this assumption creates an additional constraint without which there could be further solutions. Perhaps part of the reason it doesn't sit well is aesthetic. In the 3 + 3 + 5 = 11 puzzle, 3 doesn't divide THREE and 5 doesn't divide FIVE.

Lastly, there is a typo in the final puzzle of the Brain Fun page. That puzzle should be
TEN x TEN = FIFTY + FIFTY

Thursday, February 16, 2017

More Man Who Counted (gaps and notes)

As previously mentioned, we have been reading The Man Who Counted. While the story is good and there are nice math puzzles, we've found some of our best conversations have come from errors or weaknesses in the book. Here are three examples:

How old was Diophantus?

In chapter 24, we encounter a puzzle to figure out how old Diophantus was when he died. In summary, the clues are:

he was a child for 1/6 of his life
he was an adolescent for 1/12 of his life. (J1: "what's that?" J0: "a teenager")
childless marriage for 1/7 of his life
Five more years passed, then had a child
The child got to half its father's age, then died.
Diophantus lived for four more years

Perhaps we are wrong about our interpretation of the clues, but we noticed two things:
(a) the answer is not a whole number of years.
(b) the answer given in the book doesn't fit the clues.

For the first part, it seems a natural assumption of these types of puzzles that we are only working with whole number years. Sometimes, this is an interesting assumption to directly challenge.
Here, since the clues involve a second person (Diophantus's child) we felt whole numbers were a strong assumption. Also, the name Diophantus, you know?

Each clue required some discussion for us to agree on the interpretation. The one that seems most open is the fifth clue. In particular, did the child live until its age was half of the age of its father at the time of birth or to the point that, contemporaneously, it was half its father's age?

For completeness, I'd note that neither interpretation matches the book's answer. The first interpretation does allow a whole number answer, but it doesn't give whole numbers for all the listed segments of Diophantus's life.

Just so you can check for yourself, the solution given in the book is 84 years old.

How do you fix it?
We discussed several possible fixes:

accept answers that aren't whole numbers or require whole number segments for each clue. This allows us to take the alternative interpretation of the fifth clue (though that still isn't satisfying) or to accept the clues and just take a new answer. This isn't satisfactory because... Diophantus.
Change clue 4 or clue 5 to match the book's answer. This approach seemed to fix the puzzle without distorting it or changing the mathematics required to analyze it.
Change clue 1, 2, or 3. While possible, these seemed to open the possibility of changing the character of the puzzle. Also, these fractions were plausible based on our own experience of human life spans.

Of course, an even more satisfying answer would be to introduce a further variable and make the puzzle into one that makes heavy(ier) use of the integer restriction.

Clever Suitors

In chapter 31, Beremiz is confronted by a nice logic puzzle. Three suitors are put to a test, each is blindfolded and has disc strapped to his back. The background of the discs: other than color, the discs are all identical, there are five to choose from, 2 black and 3 white.

The first suitor is allowed to see the colors of the discs on the backs of his two competitors, then required to identify the color of his own disc and explain his reasoning. He fails and is dismissed.

The second suitor is allowed to see the disc on the back of the third suitor, then required to identify the color of his own disc and explain his reasoning. He fails and is dismissed.

Finally, the third suitor is required to identify the color of his own disc and explain his reasoning. He succeeds.

Weakness 1
As a logic puzzle, we enjoyed this. Our problems came from the context in the story. This challenge was set to the three suitors as a way of fairly judging between them by finding the most clever suitor. However, this process was clearly unfair. In fact, it is inherent in the solution that it was impossible for the first and second suitors to determine the color of their own discs.

This led to a nice discussion about who really held the power in this process: the person who structured the problem by deciding what color disc should be on which suitor and what order they would be allowed to give their answers.

Extensions:

consider all arrangements of discs. Are there any arrangements where none of the suitors can answer correctly?
What is the winning fraction for each suitor? If you were a suitor, would you prefer to answer first, second, or third?

Weakness 2

Our second objection was non-mathematical, but again related to the story context. The fundamental problem wasn't how to choose a suitor. The fundamental problem was how the king could remain peacefully friendly toward all the suitors' home nations through this process.

For this discussion, we went back to the story of Helen of Sparta, which we'd read a long time ago in the D'Aulaire's Book of Greek Myths. Of course, that also led to discussion of the division of the golden apples, another puzzle we all felt surely could have been solved more effectively with some mathematical reasoning...

The Last Matter of Love

The last puzzle of the book is in chapter 33. It is another logic puzzle, again intended to test the merit of a suitor in marriage. The test:

there are five people
two have black eyes and always tell the truth
three have blue eyes and always lie
the suitor is permitted to ask three of them, in turn, a "simple" question each.
the suitor must determine the eye color of all five people

As a logic puzzle, we readers get some extra information:

The first person is asked: "what are the color of your eyes?" The answer is unintelligible.
The second is asked: "What did the first person say?" The answer is "blue eyes."
The third is asked: "What are the eye colors of the first and second people?" The answers are "the first has black eyes and the second has blue eyes."

Simple questions

Our first objection was the part about asking "simple" questions. Having developed our taste for these types of puzzles through the knights and knaves examples of Raymond Smullyan (RIP, we loved your work!!!), the third question really bothered us. If you're going to go that far, why not ask the third person for the color eyes of all five people?

Personally, I would prefer that the puzzle require us to ask each person a single yes/no question.

As an extension: can you solve the puzzle with that restriction?

Getting lucky

Again, we felt that this puzzle didn't meet the requirements of the context: to prove the worthiness of the suitor. Putting aside the question of whether this is really an appropriate way to decide whether two people should be allowed to marry, the hero here got lucky.

Extension: what eye color for the third person would have caused the suitor to fail?

Extension: what answer from the third person would have caused the suitor to fail?

Extension: for what arrangement of eye colors would the questions asked by the suitor guarantee success?

Extension: what was the suitors' probability of success, given those were the three questions asked?

Waste

Our final objection was the simple waste in the first question. From a narrative perspective, this is justified and even seems made to serve the purposes of the suitor. However, it opens another idea:

can you solve the puzzle, regardless of eye color arrangement, with only two questions?

Feel free to test this with yes/no questions only or your own suitable definition of a "simple" question.

The power of...

As a final thought, let me say that I think errors and ambiguity in a text are a feature, not a bug. It is another great opportunity for us to emphasize that mathematics is about the power of reasoning, not the power of authority.

Wednesday, February 1, 2017

Perfect Play for My closest neighbor

Joe Schwartz at Exit10a wrote a fraction comparison post that prompted me to write up more of my experience and thoughts on this game.

Let's find perfect play
This week, I intended to use the game one last time with the 4th graders as an extended warm-up to our class. The challenge I presented:

If we got super lucky and were given perfect cards for each round of the game, what are the best possible plays?

My intention was to spend about 20 minutes on this. Depending on how quickly it went and the kids' reactions, I considered giving them a follow-up for a short homework: what are the best plays if we include all cards A (1) through K (13)?

How did it go?
In the end, the basic activity took the whole class. These comparisons were difficult for the kids, so we spent time talking about each different strategy for comparison:

common denominators
common numerators
distance to 1
relationship to another benchmark number. Like 1/2 in Joe's 4/6 and 8/18 example, a benchmark is a "familiar friend" that should be relatively easy to see it is larger than one and smaller than another. In practice, 1/2 seems to be the most popular benchmark.

For visualization, drawing on a number line seemed to work best.

I did not assign the full deck challenge as homework. Instead, we gave them some more work with fractions of pies and bars.

What have I learned?
This game is really effective at distinguishing levels of understanding:
(0) some kids are totally at sea. They don't really understand what this a/b thing means, how a and b are related, etc. These kids struggle with the first round of the game when the target is 0, when the idea is to just want to make their fraction as small as possible.

(1) Some kids have got a basic understanding of the meaning of the fraction and can play confidently when the target is 0 or 1. They might still be weak about equivalent fractions. Trying to play some spot-on equivalents when 1/3 and 1/2 are targets is a give-away.

(2) familiar with some frequent friends: kids who can tell readily whether their plays are larger or smaller than the target for 1/3, 1/2, 3/4.

(3) proficient: have at least one consistent strategy they can work through to make a comparison

(4) fraction black-belts: using multiple strategies, already familiar with many of the most common comparisons.

What would I do differently?
Generally, I think it is valuable to spend more time and more models directed at the basic understanding of what fractions mean. The kids who were at or close to stage 4 have, over the years, been seeing diagrams of pies, cakes, chocolate bars, number lines and physical experience with baking measures and fractional inches on measuring tapes and rulers. Oh, and also actual pies (mostly pizza), cakes, cookies, and chocolate bars discussed using fractional language.

More locally, for this game in a class of mixed levels, I would

lean toward doing this more as a cooperative puzzle
re-order the targets for the rounds as 0, 1, 1/2, 3/4, 1/3, 2 (note: I don't have strong feelings about where 2 fits in this sequence)
I also would consider allowing equivalent fractions to the target as winning plays

Tuesday, January 10, 2017

Running, rates, rounding

My running session this morning gave me an idea for a kind of 3-act math discussion with J1 and J2. I will discuss this with them when they come back from camp and see what they think. I expect the last questions will be hard for them and I would like to see how much progress they can make working together.

First Act

Today, I went running and recorded some information on my GPS. For five laps, I ran moderately fast. Here is the data:

Time	Rate	Distance
3:00	12.7 kph	635 m
3:00	12.9 kph	647 m
3:00	12.6 kph	633 m
3:00	12.7 kph	637 m
3:00	12.8 kph	645 m

What do you notice?
What do you wonder?

Second Act

My target was actually to run 12 kph for each of these three minute segments. After the first lap, I knew that I could run more slowly and still hit my target. I wondered, how much less than 635m could I run and still hit my target?
If I compare two laps, both rates and distances, can I figure out the distance I get for each 0.1 kph? Is there another way to calculate the difference in distances for each 0.1 kph?

Third act

For some reason, this made me think about rounding that J1 had recently been studying. He is a bit disturbed about what to do with values that are halfway between the rounded levels, for example whether 15 should round up or down to the nearest ten. Since this investigation of running data involved calculations with measured values and rounding, I though it would be instructive to explore a couple of calculations:

I have two distances, rounded to the nearest 10 cm of 20 cm and 10 cm. What is a reasonable range for the difference of those distances?
My GPS measured a time of 3 minutes (3:00, rounded to the nearest second) and speed of 12 kph (12.0 kph rounded to the nearest tenth of a kilometer per hour). What distance did I run? What is a reasonable range for that distance?

Monday, December 5, 2016

Leftorvers with 100 game

In Grades 3 and 4, we played a nice game that (I think) we got from Marilyn Burns. Looking for a reference after the fact, I see it explained in her book Lessons for Extending Division.

Basic play

Start with a target number (we used 100) and collection of available divisors (we used integers 1 to 20)
Players take turns choosing a divisor from the remaining available options. They divide the current target by that divisor and keep the remainder as their score for the turn. They also subtract the remainder from the target to create a new target for the next player.
Each divisor gets crossed out when it is used, so it can only be used once.
The game ends when the target is reduced to 0 or when all available divisors are exhausted.
We played as a two player game.

Here's an example of a game play:

Player 1 chooses 17. 100 = 17 * 5 + 15, so player 1 scores 15 points, the target is reduced to 85, and 17 is no longer available as a divisor.

Player 2 chooses 20. 85 = 20 * 4 + 5, so player 2 scores 5 points, the target is reduced to 80, and 20 is no longer available as a divisor.

Player 1 chooses 14. 80 = 14 * 5 + 10, so player 1 scores 10 points, the target is reduced to 70, and 14 is no longer available as a divisor.

Player 2 chooses 18. 70 = 18 * 3 + 16, so player 2 scores 16 points, the target is reduced to 54, and 18 is no longer available as a divisor.

Player 1 chooses 19. 54 = 19 * 2 + 16, so player 1 scores 16 points, the target is reduced to 38, and 19 is no longer available as a divisor.

Player 2 chooses 13. 28 = 13 * 2 + 12, so player 2 scores 12 points, the target is reduced to 26, and 13 is no longer available as a divisor.

Player 1 chooses 15. 26 = 15 * 1 + 11, so player 1 scores 11 points, the target is reduced to 15, and 15 is no longer available as a divisor.

Player 2 chooses 16. 15 = 16 * 0 + 15, so player 2 scores 15 points, the target is reduced to 0 and the game ends.

Player one wins 52 to 48.

Our experience

We found this to be a fun, interesting, and engaging game. The practice with dividing and remainders was pretty obvious. In addition, it opened up some opportunities for strategic thinking, particularly at the end-stage of the game. I think there are also several good extension explorations.

Extensions

First, I created a simple pencilcode program for two players to play this game against each other. Here's a playable version (and here's the code).

Second, you'll notice that the first player in our sample game followed a "greedy strategy." At each stage, that player chose the divisor that would give the most points on that turn. If you look closely, that isn't the best strategy at the end of the game.

So, a natural exploration is to find the best strategy for different starting targets. One specific point of about which we're curious: is it ever desirable to skip your turn (choosing 1 as the divisor is effectively a turn skip)?

Some other areas for investigation:

must the game always end on 0 or can we run out of divisors?
given a target and collection of starting divisors, what is the shortest (number of turns) game possible? What is the longest game (number of turns) that does end at 0?

Sunday, October 2, 2016

Vacation Plan: Emotional Skills

This month is a school vacation period for the three Js. One area of focus this month will be on emotional intelligence skills.

Component Skills

We found a nice overview on Psych Central. We talked through the first four with J1 and J2 to start the month:

Self-awareness: (a) recognize your own emotions and their effects, (b) sureness about your self-worth and capabilities
Self-regulation: using a number of techniques to alleviate negative emotions
Motivation: tools to manage motivation to achieve goals.
Empathy: discerning the feelings behind others’ signals

The article also includes Social Skills as a category, but these seem separate to us.

Around the discussion of empathy, J1 asked how it differs from sympathy. We think that a difference is understanding how other people feel and their perspective (empathy) vs sharing their feeling (sympathy). I admitted that I do not have much sympathy.

A vocabulary list

Underlying many of these skills is a vocabulary of emotions. We found two nice resources for this:

Emotions color wheel: this is a great visual for the kids.
Vocabulary list for greater shading: the idea is to move beyond the standards, happy, sad, angry, to get more shading and nuance. Another hope is that, in the moment of analyzing the emotion and comparing with the vocabulary, it will help their self-awareness and provide a point of detachment from the emotion.

Our Focus

J1 chose to focus on skills relating to empathy and sympathy.

J2 chose to focus on skills related to self-regulation.

Daily Schedule

Supporting this skill development and general household organization, we are posted this schedule for the month. You can probably tell that the boys helped write the schedule:

Things to do everyday:

3 pages of Beast Academy and discussion with J0
10 minutes of spelling with P
Vocabulary: writing a sentence and 5x words (5 words/ day for J1, 3 words/day for J2)

07:00 Wake up
urinate
put dirty nightclothes in basket
shower
get dressed

08:00 prepare breakfast
eat breakfast
brush teeth
poop

12:00 help with lunch
eat lunch
clear table from lunch

17:00 help with dinner
eat dinner
18:30 clear table from dinner
practice music
put dirty clothes in basket
urinate
bath
brush teeth

20:00 get in bed
listen to story
20:30 lights out

Sunday, July 24, 2016

Beast Academy and Dreambox (reviews)

Conflict of interest statement: I do not have a current or pending financial relationship with Art of Problem Solving, but I have several friends on their board and have had direct contact with several other people there. We purchased and currently own all of the books I review below.

I have no relationship with Dreambox. We tested the program using their free trial and then paid for a 6 month subscription.

Beast Academy

What is it?
Beast Academy (from Art of Problem Solving) is a book series with 10 "guidebooks" and 10 parallel "practice" books targeted to 3rd, 4th, and fifth graders. Note that the first of 4 books for fifth grade has only recently come out and they are planning to extend to 16 x 2 books covering 2nd to 5th. We do not have either 5A or 5B yet.

We have read all 8 books from 3a to 4d; J1 and J2 have gone through practice books 3A - 3C.

While these are not math exercise apps, I'm going to borrow some of the elements I've used in past app reviews. One key point I want to emphasize for both books and apps: the way you use them can also determine their benefits or costs.

TZB3
To drive this point home, let's start with Tracy Zager's Big Three criteria (see here):

1. No time pressure: Neutral since this is really up to you, parents.

Do you set a timer when they start a page of practice or a question? Do you require a certain amount of time spent on math practice? While the books do not suggest or impose a sense of time pressure, there are story segments involving math competitions that imply speed is important.

One time element that is and has always been great about physical books is that they sit around. This means they are available and tempting. Almost every day, there will be someone flipping open one of the BA guidebooks, even J3 for whom the material is too advanced right now.

2. Conceptual basis: yes (pass)

The books introduce models, contexts, and conceptual ways of considering problems and techniques.

3. How are mistakes handled: again, this depends on you and your kids

My approach is to go through the problems and select ones to discuss. I don't use the answer key, so I do the problems myself. This means we have three categories of questions to discuss (a) answered correctly and I found interesting, (b) answered incorrectly, (c) answered correctly by the kid, but I made a mistake.

Also, I am very positive in how I talk about mistakes. The key message is that these are actually the best learning opportunities and create a chance for us to understand our own thinking.

Preliminary summary: whether Beast Academy (or any printed material) passes the thresholds depends on how you plan to use it. If you want to deviate from Tracy's guidelines, either adding time pressure or incentives based on minimizing mistakes, you probably should think carefully about whether that's wise.

The good

For my kids, the stories and themes in the guidebooks hit the right tone. They are engaging and funny, with a humor that is occasionally silly or corny. An extended quote from The Princess Bride certainly wins some extra points as well. More bonus points for becoming, via malapropism, the source of J3's current catch-phrase, "I get it: pointillism!"

For me, the organizing theme of the material seems to be "ideas you encounter when playing with math." In some cases, the exercises create "aha moments," like when J1 realized he didn't always have to calculate side lengths of a polygon to use his knowledge of its perimeter in a challenge. In other cases, like calculating (n+1) x (n-1) there are interesting patterns to notice and connections to make.

I'd note that the workbooks are absolutely essential as there is a lot of material that is introduced in the context of exercises. I think these books are excellent, well selected, well sequenced, with enough repetition to facilitate mastery and enough variation to avoid boredom. In fact, I really enjoy doing the problems myself.

Overall, we find the practice books an especially good source of cues for quick (5-15 minute) math conversations.

The Bad
Any worksheet-based system is weak in generating exploration and deeper investigation. Beast Academy partially addresses this by including open-ended games and an occasional investigation. While nice, this point remains a weakness. I don't want to belabor this point, since it is not a unique problem with Beast Academy. Indeed, I think it is a universal issue with static educational material.

Unfortunately, the only solution I know is to involve a human guide. Fortunately, I am able to play that role, asking their thoughts about interesting problems, helping them form connections with earlier or other material, getting them to follow useful side-branches or to continue more deeply into a particular area.

Eventually, of course, we hope to develop enough mathematical habits of mind that the kids will do these things on their own. Realistically, I don't think that will happen until they are well clear of any elementary age material!

The Ugly
I don't see any fatal flaws in Beast Academy.

Grand Summary
If you can use the material the way we do, I highly recommend Beast Academy.
If you can't or don't feel comfortable engaging as your kids' mathematical guide, these books are probably still one of the best options. Just don't set up a timer and demand perfect answers to all the questions!

Dreambox

Dreambox is a math facts, basic skills system. It has material from pre-school through high school. We have spent a lot of time with the elementary grade material and a little sampling of the high school content.

TZB3
Dreambox was one of Tracy Zager's positive examples in her app post, so we already expected it would pass these three criteria. After spending so much time with the system, though, we've seen that not all activities within DreamBox completely satisfy the checklist:

1. No time pressure
Some activities do include time pressure. For example, there are a family of "games" around multiplication automaticity where a collection of calculations stream across the screen. This really does raise the stress level for kids.

In a slightly different form, there are other activities involving virtual manipulatives that require the student to do something using the minimum number of moves. Like the time pressure, this seems to create confusion where the kids can get something right, but still get it wrong.

2. Conceptual Basis
I mostly concur with Tracy's original assessment. Almost all activities have a conceptual component. The timed calculations mentioned above don't, so those get a double demerit.

3. How errors are handled
Again, mostly agree with Tracy. However, there are some activities where, for a minor mistake, one is required to redo a number of manipulations, rather than fix the earlier work.

The good
The underlying math curriculum here is solid, if basic. The clear strength of this system is the pictorial representation of manipulatives offering models that build number sense, reflect operations, and show place value. In the early years section, where we have been spending most of our time, almost every activity is based around one of the manipulatives.

The other thing Dreambox does well is present a sensible progression for the different activity streams. I think this works especially well for J3 who is going through much of the material for the first time. As she encounters a new formulation, she will study it for a while and then there is a clear moment when she has figured out the new complication.

I'll give two examples. For J3, there is an activity to replicate a number bead pattern and then click the number of beads in the arrangement. Her primary tool is to count the beads one-by-one. In the most recent module, she gets a short view of the arrangement and then it is hidden (it can be revealed again, if you choose). This is forcing her to build new skills, either memorizing the arrangement to mentally count or a more advanced counting technique.

For J2, one of the place value exercises involves grouping items into pallets (1000s), cases (100s), boxes (10s) or loose items (1s). The current module asks him to consider multiple different ways to pack a given number. For example, 1385 items could be packed in 1 pallet, 3 cases, 8 boxes, and 5 loose items, or 13 cases and 85 loose items (among many other options).

One other strength of DreamBox is the email feedback to parents. Christopher Danielson recently noted this in a post: Parent Letters.

The Bad
I have seen three areas of weakness with Dreambox: the way mathematical tasks are presented, the pace of adaptive adjustment, and the absence of rich tasks. I'll talk about each of these in turn.

The theme gives an irritating appearance of choice. For example, in the early elementary section, the kids can play with dinosaurs, pirates, pixies, or animals. Under each of these, they have a further choice about what story to explore. Those choices, at least, lead them to different narratives and animated sequences.

At that point, all of the stories involve finding missing items. Users then see another choice asking where in 6 map regions they want to look for the missing items, but this isn't really a choice as there are no differences between regions and they will have to go through each region eventually.

Similar to Prodigy Game, the math tasks are presented as an annoyance to be overcome, the cost the student has to pay to move on with the story. Again, I find this creates unfortunate subtext to the mathematical experience.

Second, the adaptive adjustment is very slow, if it actually exists. In their FAQ, I see that they get questions about how to increase the challenge level, so this seems to be a common experience. Part of the problem is that they intentionally start students with material below their grade level.

Finally, the tasks in Dreambox are basic. While they may present a challenge for a new learner, as J3 is experiencing, they should eventually become so easy that they are boring. In some way, this feels like learning to solve math class tasks without having to develop or use any mathematical habits of mind. Further, the thrill and fun of playing Dreambox lies in unlocking the animated stories and collecting tokens, not in doing math.

For J1 and J2, this thrill has worn off after about 2 months with the system.

The ugly
Nothing in Dreambox is a show-stopper.

Summary
Properly understood as a basic curriculum substitute or source of practice exercises, Dreambox is a solid application. Just don't make the mistake of thinking it will either foster a love of math nor deeper mental habits.

*Update* A quick comparison with ST Math
I was sitting on this review, partially written, for a long time. One thing that got me to finalize the review was going through the demo challenges on ST Math with J2. We had previously tested ST Math many years ago with J1 and it was really good. Once again, this is what I saw with J2: really cleverly presented scenarios that gave us good models for the math and a really fun user experience. After playing for about 30 minutes, J2 said: "this is a lot more fun than DreamBox."

If I can get a subscription, we'll test it more extensively and write a review to see whether that really holds up.

Wimbledon game

We have been playing the game Wimbledon from John Golden. J1 and J2 absolutely love the game and J3 has enjoyed pretending to play as well. Definitely try it out!

Below is a session report sharing some of our experiences with the game, including some alternative rules (aka mis-reading).

Our basic play
For the serve, we allow the following options:
(1) serve a single card from the top of the deck
(2) serve one or two cards from your own hand

We also allow returns that have the same value, if the largest card in the combination is higher. For example, 8 + 2 can be played on top of 7 + 3.

When playing doubles, we followed tennis conventions: one player serves the whole game, return of serve alternates. For the return of serve, the designated player must return on their own. For other returns, either of the partners alone or in combination can play cards for a return.

Having gone back to John's original post, I now see that we played with the inverse rules for aces. On the serve, we counted them as 11, all other times 1. We didn't distinguish between aces played from the hand or served from the deck, those were all 11s. That formed a strong advantage for the servers, while making aces essentially worthless for all other players.

With three players, I had the kids play as partners and I played with a ghost partner. The ghost partner would contribute cards randomly. When the ghost played cards that weren't large enough to be legal plays, we considered that an unforced error and awarded the point to the other team. While the ghost was able to hold serve for one game, it was a big disadvantage. An alternative for three players would be to have the ghost partner with the server and for everyone to take turns serving. This would put the server advantage (with our "house rules" for aces) against the ghost disadvantage.

A modification
In our play, we have found that the 10 value cards and aces (on serve) dominate game play. Here are two ideas to address that:

Assign face cards values 11 for Jack, 12 for Queen, and 13 for King. Ace, on serve, can have a value 14.
Allow players to combine as many cards as they like. This would probably work best with our "Further extension" rules below. A possible sub-variant is to only allow gradual escalation where the players can step from single card plays to 2 card plays, from 2 to 3, etc, but could not jump from a single card play to a 3 (or more) card play.

My instinct is that the variation we will like the most is to differentiate the face cards and allow gradual escalation for multi-card plays.

Further extension
Now that we've gotten comfortable with the basic and doubles games, we are considering a more complex version. The idea we are considering is to somehow limit the players' abilities to refresh their hands by redrawing so that burning a lot of cards will have a cost.

This is the rule modification, written for a 2 person game:

Create a draw pile for each player with 15 cards.
At the start of the game, each player draws 5 cards into their hand.
Points are played as in the normal rules
At the end of a point, the players refresh their hand up to 5 cards from their draw pile
If a player runs out of cards in their draw pile, they cannot draw additional cards to refresh their hand.
If both players run out of cards in their draw pile, then shuffle the pile of face-up played cards and give each player a new draw pile with 10 cards.
Repeat step 6 as often as needed

The idea of this variation is to thematically mimic the idea that one player could push too hard, too fast, and get tired out relative to the other player. Strategically, our idea is that this will also create a tension between dumping your own low cards and letting the opponent dump.

Monday, June 20, 2016

Secret Numbers (Addition Boomerang variants)

We have been enjoying Mathpickle activities in our math games classes recently. This week, the 3rd and 4th graders will be playing with some of the more advanced Addition Boomerang variations.

During our planning, we came up with one extra pointer to tie the activity more closely with multiplication. Also, we had ideas for variations and wanted to record our notes so we can use them again in the future.

Tie with multiplication

In Gord's video explanation, he sometimes records in the center of each loop how many times that loop has been used. We emphasize this and write some related equations to help draw out the connection between the repeated additions in this activity and multiplication.

The first way we do this is by making tally marks inside the loop every time that branch is chosen. At any time, you can pause and write down an equation for the current total in a form

AxN + BxM = Total

where A and B are the values of the loops, N and M are the number of times each loop has been used.

Alternatively, we can show a "completed" round of throws by simply writing the number of passes for each loop in the middle and, again, write out an equation showing the total as the sum of two products.

Secret Number Variations

We start with a basic addition boomerang lay-out, either with 2 or 4 branches, both players (or teams) share a common set of addends and take turns adding on to a common running total. In our variants, the players choose and write down a secret number that helps inform their target for the game:

Version A: players pick a number between 70 and 100. This is their target for the game and they win if the common total hits that value, whether the target is reached on their turn or their opponents turn.
Version B: players pick a number larger than 15. They win if the total hits a multiple of their secret number. For example, if they choose 17 and the running total hits 51 (aka 17 x 3) then they would win. If the total is a multiple of both secret numbers, then the player who chose the larger secret number wins.
Version C: players pick any number. They win if the total hits a multiple of their secret number that is larger than 60 (not equal to 60). If the total is a multiple of both secret numbers, then the player who chose the larger secret number wins.

Version B is, I think, the most directly playable.

Possible issues
I'm not sure how to deal with the case where both players choose the same secret number.

For Version A, it will be interesting to see what modification kids can find that will deal with the fact that it is very easy to miss any particular target. In the basic game, once the total is larger than your target, there is no hope of recovery. There are several ways to address this. I would be eager to hear any rule sets that kids create and hear about the experiences.

In Version C, I wonder if choosing 2 as the secret number is too strong a move?

Monday, June 6, 2016

Broken Ruler and Multiplication refresh

Ruler Explorations

We noticed that one of our tools, a ruler, had gotten broken.

Is it still useful? As a challenge, J2 looked at measuring a noodle from his soup.

There were two ideas:

the noodles were too long, so had to be broken in pieces to measure with the remaining ruler
Our ruler doesn't have to start at 0, we can use subtraction!

While we were talking about this, I recalled the idea of Golomb Rulers. We came up with a ruler that was marked only with 0, 1, 3, 7, 11, 12 cm. This lets us measure 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12cm distances.

What if this ruler gets broken? For example, we imagined cutting our ruler between the 1cm and 3cm markings. What measurements are still possible? Is there anything interesting in the relationships between how many ways there were to measure a distance before the break and how many ways to measure after?

Multiplication Refresh

We recently re-watched Graham Fletcher's Progression of Multiplication. Both J1 and J2 did some practice around this. The most interesting point was J2's reaction to Graham's comment at 4:54: "This sucks!"

"Why did kid's say that?" "Hmm, let's try out a couple of examples..."

We rolled dice to randomly generate digits for an example and were lucky to get 35 x 34. J2 quickly saw this as 35 x 35 - 35 and knows a pattern that let him quickly calculate 35 x 35 = 1225. As a result, 35 x 34 was pretty easy for him to calculate.

Then, he worked through a graphical representation and a powers-of-ten version. At the end, we got to compare and contrast the different approaches.

Continuing to play with some old activities

Fold-and-punch
We did some more fold-and-punch activities. This time, we folded the paper, then drew a location for the punch, and tried to figure out how many holes would result and where they would be. We broke out our serious hole-puncher:

Unfortunately, must be operated by an adult

In this example, we got a small surprise that the result wasn't a power of 2:

Chairs (and tables)
Another round of building chairs, following the NRICH activity. This time, with J3:

We got to compare and contrast our designs:

how many cubes were used for the legs? Which one had more and how many more?
How many cubes were used for the whole chair? How did they compare?

Monday, May 30, 2016

Class summaries

Quick notes on the activities for the classes today.

Grades 1 and 2

Mobius loops
We are splitting up games with other activities. This week, we are introducing several geometry explorations. First up are Mobius strip activities, nicely shown in this Matt Parker second favorite shape:

cylindrical loop: using 2 colored pencils, draw a line along the center of the loop on the inside and the outside. We can see that there are two sides, no big surprise. Cut along one of the lines and the loop splits into two new loops
Mobius strip: give the paper a twist. Now, draw along the middle of the paper and see that there is only one side. Now, cut along this line and see what happens. Repeat this, drawing another line along the center of the new strip. Do you have one side or two? Again, cut along the central line. What do you get?
Two connected loops: tape two, untwisted loops, together in perpendicular directions. Now cut along the center lines of each loop. What do you get?
Two connected Mobius strips: tape together two mobius strips and cut along their center lines. What results now? Did everyone get the same result?

There are some natural extension explorations:

try these with more twists (as per Matt's video)
keep cutting the center lines
Connect a Mobius strip and an untwisted loop (half-way step between 3 and 4). Now, cut along the center lines. What happens?
Inspired by the thinner and thinner loops, kids can explore ways to cut paper so that they get longer and longer strips or loops

Note: these activities can be even more rewarding when something goes wrong. For example, what if there isn't enough tape connected the ends of the loops? These mishaps make everyone pause and consider more carefully what is actually happening.

Also, in the class, we only had time for the first two make-and-cuts, then demonstrated the two connected loops.

Punch (fold and cut)
All this cutting fits nicely with our second exploration: the punch activities from Joel David Hamkins' post punch, fold, and cut from Joel David Hamkins.

Grades 3 and 4

We started with the Shapes x Shapes puzzle from NRICH:

We added a couple of extra questions to this challenge:

Before completing the puzzle, which numbers do they think are excluded? Why?
Make extra equations that allow us to include those missing numbers. Are they easier to incorporate using multiplication or addition equations? What about equations that combine multiplication and addition?

Observations:

Once again, this appears to be a very simple activity, but gave us a lot to talk about. In particular, it was very helpful for highlighting a lot of misconceptions and gaps in understanding. Examples:

"identity" relationships were still unclear: 1 x n = n, 0 x n = 0
Several students thought the first equation would be 4x4x4 = 12 (confusing multiplication and addition)

Division Dice move to Cards
For our core activity, we are extending the Division Dice game. This time, we use playing cards, A through 10, instead of dice rolls to generate the random components of their equations. In this case, the aces are wild and can be any number from 2 to 10. When they form a multi=digit number with a 10, the 10 counts as two digits. For example, 3, 5, 10 could form 105 ÷ 3.

With cards instead of dice, we lack the natural move of flipping the dice to the opposite side which we used to make sure all throws could give us whole number divisions. In this version, we allow division with remainder. However, the twist is that the remainder becomes points for the opponent.
For example, if I draw 3, 3, 5, I can form 53 ÷ 3 to score 17 points for myself, but the opponent gets 2 points.

Question: Are there cases where the best play is not to form the largest possible number divided by the smallest number?

An interesting game variant: swap the scoring so that the active player scores the remainder and their opponent scores the quotient.

Tuesday, May 24, 2016

Logic Puzzle collection and Dropping Phones

Recently, Mathbabe put out a request for riddles. There are some good links in the comments:

The Riddler on FiveThirtyEight
Logic Puzzles on Math is Fun
Alex Bellos at The Guardian
Riddles.com (naturally)
Puzzling.stackexchange.com (is there a stack exchange for everything now?!)
Authors cited: Dudeney, Gardner, Polya, Smullyan, Vaderlind.

We've played with puzzles from almost all of these sources in the past, but this was a good opportunity to put together a nice list.

The one that stood out to me was on FiveThirtyEight. That's a site I read frequently, especially during this US election season ... and I was totally unaware of The Riddler feature. We kicked off with the oldest puzzle from their archive: Best way to drop a smartphone.

Breaking stuff

Right away, J1 and J2 loved the theme and were into questions about whether they could somehow keep the phones, if they weren't broken, or utterly destroy them, if they were broken. We talked about setting an upper bound with a very simple strategy of starting on the 1st floor and working up each floor. That's not a great answer, but it got them into modifications and improving strategies.

Through the conversation, it was interesting to see them start with the idea that the drops for one of the phones would be de minimis and could be ignored, but then start to pay attention to that aspect. Also, they had to grapple with the idea of balancing the number of drops that would be required in different cases.

While we didn't get to the optimal strategy, but the kids managed to get a version that, at worst, would take 19 drops for the 100 story building. Their intuition was based around taking the square root of 100. They could see that this probably wasn't the best answer, since there were still cases that, at worst, would take 10 drops and others that, at worst, would take 19.

Can you do better?

Smaller before bigger

The puzzle page poses the same challenge for a 1000 story building. However, we found something interesting when working on the version for a 10 story building: there is strategy where all worst cases take the same number of drops, but it is not the optimal strategy!

It is a little hard to write about this without disclosing the strategy, but here's a hint:

4 + 3 + 2 + 1 = 10 and 5 + 4 = 9
We are always allowed to assume that the phones will break if dropped from the top floor of the building

This led to another extension: what size buildings will have the same issues as for a 10 story building?

Probability comes in

Another extension is to think about the expected number of drops required and strategies that minimize this. Crucially, this extension introduces the idea about our prior beliefs about the sturdiness of the phones: where do we think the phones are likely to break, what is our confidence?

We didn't pursue this extension very far, but it did lead to some interesting conversations about terminal velocities. For those who want to follow that thread, this (other) stack exchange thread might suit you: How to figure out height to achieve terminal velocity.

Monday, May 23, 2016

Division Dice (math games class)

Who: grades 3 and 4
Where: in school

A dice division game

We created a simple dice game to practice division. Here's a description of the basic element of play:

roll three dice: for example, 3, 4, 5
group two of them into a two digit number: for example, 45
Divide the two digit number by the remaining single digit: for example, 45 / 3 = 9
This value is your score for the round
First player to 200 or more points wins (we used 100 for the initial game)

Key constraints

You can only score points if the single digit is a factor of the two digit number (remainder must be 0)
Where there are multiple options, the player can choose the combination that gives them the maximum score

This pencilcode program (see code) analyzes this basic game structure, identifying how often there will be no legal scoring arrangement and showing a histogram of the largest scores.

Modifications/Extensions

I wasn't satisfied with three elements of this game: (a) any time a 1 occurs, the division calculation is too easy, (b) too many combinations don't allow a score (about 15%) and (c) there aren't many decisions for the students to make (just six combinations to investigate).

We addressed these by adding two extra rules:

1 is a wild that must be replaced by a value from 2 to 6 (cannot be left as a 1)
On your turn, you can flip the over the dice in the two digit number. For example, a 6 can be flipped to a 1, 5 to 2, 4 to 3, etc.

The first point removes the division by 1 cases, the second one allows more choices and reduces the number of non-scoring cases.

*UPDATE*
Allowing the dice flip and wild 1s seems to make the game too loose. Instead we dropped the wild 1s rule and added these two:

Division by 1 is not allowed in the game
If you roll triple 1, re-roll