Sunday, January 11, 2015

An elementary extension attempt

Who: J2
When: just after brushing his teeth for bed
Where: bedroom

A while ago, J2 realized that adding consecutive odd whole numbers would give him squares. For example, $1+3+5+7+9 = 25.$

He loves to ask variations on this sum of odds question. Tonight, he came up with a new idea:

what is the sum of odds up to 10?

We discussed and he clarified that he wanted 10 included somehow, not just the equation above. How would you extend the idea of adding consecutive odd numbers?

Our idea
The odd sum and squares relationship looks like this:
$$ 1 + 3+ \cdots + (2n - 1) = n^2$$

This isn't how J2 thinks of it yet. He focuses on the largest term $m$ and then, through sequential calculations, forms $m+1$, then $(m+1)/2$ and then $\left(\frac{m+1}{2}\right)^2$.

His idea tonight for answering his own question was to apply the same procedure (formula) to 10. In essence, what he was doing was saying:
  • I know what it means to "add odd numbers up to $m$" when $m$ is odd.
  • I know that the result is $\left(\frac{m+1}{2}\right)^2$ in that case.
  • I can then use that as a definition of what it means to "add odd numbers up to $m$" when $m$ is not odd!
I helped with the final squaring calculation, giving the answer: 30.25 is the sum of odd integers up to 10.

Credits
My ability to now include $\TeX$ equations is thanks to Sachin Shanbhag and Steve Holden.

Saturday, January 10, 2015

Money game and some cute little challenges

Who: J1
When: after breakfast (no school today!)
Where: dining room table
What you need: playing cards with images of money on them

Played a cute game called '51' from RightStart Math using their US coins card deck. It was an ok game that led to a surprisingly interesting discussion.

The game in brief:
  • The US coin cards show the obverse image from pennies, nickels, dimes, quarters, and Kennedy half dollars. One side of each card has two copies (two total images) of the coin, the other sides are a common pattern (so you don't know the coin when the card is turned down).
  • Players are each given 3 cards
  • On each turn, a player plays a card to an open stack making a total less than 52 cents, collects the stack if they have made exactly 51 cents, or plays a card to create a new stack if their coin would cause the existing stacks to overrun 51.
  • At the end of the game, compare who has collected the most cards
An ok game with some strategy and some calculation practice (particularly reinforces the strategy of regrouping and number facts related to coin denominations).

Here was the fun part:

Challenge 1

At the end of one game, we had 90 cents unclaimed on the table.


I was initially surprised, thought for a moment, then realized what had happened. Questions I posed to J1 that I offer to you:
  1. Why was I surprised?
  2. How much did we usually have remaining on the table at the end of play?

Challenge 2

How many dollars are represented by coins in our card deck? Hint: it is an integer.

Challenge 3

What is the maximum amount that could be stranded at the end of the game? Does this depend on the number of each denomination in the pack?

Friday, January 9, 2015

Stairway to (number) heaven

Who: J1, J2, J3
Where: our stairs
When: permanently (ha!)

J2's school has a set of stairs used by the kids with numbers posted on each step. I finally realized we could copy this at home. It has been great fun for each of them, in slightly different ways:
  • J3 feels like these are "her" numbers. Everyone else has told her we made them for her and the sense of ownership makes her pay a lot of attention to them.
  • J2 really likes having numbers and shapes posted around the house, so he is the one who is encouraging us to continue to the top floor.
  • J1 likes taking J3 on a tour of the numbers and admiring the pictures. He also likes coming up with new things to add (like writing a related number expression)
  • They all like talking about the ones they made and their contribution to the project
I have three recommendations for you replicating:
  1. Start with zero
  2. Be open to a lot of interesting number and shape conversations while making the cards.
  3. Do it! if you don't have stairs, make it into a horizontal number line on the walls. 

J2 almost finished decorating the 12 card

Cards in situ, ready to be enjoyed

Detail of the 8 Card: such craftsmanship!

Wednesday, January 7, 2015

Pandemic: different types of randomness

who: J1
where: my home office
when: after lunch

We've been playing pandemic a lot recently.


For this post, you don't really need to know the game, just that there are a deck of playing cards from which each player draws on their turn. Usually, these are resources that are needed to win the game, but occasionally there are epidemic cards that really stink. Oh, also know that everyone is playing together, cooperatively, against the game itself.

J1 noticed something very interesting by varying the set-up:
  • Normal set-up: split the playing cards into equal piles and put one epidemic card into each pile (number of piles is the number of epidemic cards you are using). Shuffle those piles and then stack them on top of each other.
  • Modified set-up: put the epidemic cards you are using in the deck and then shuffle the whole deck.
Focusing just on the epidemic cards, these result in very different distributions. Here are some challenging questions (assume there are 5 epidemic cards and 50 non-epidemic playing cards):
  1. For both distributions, what is the probability that the n-th card in the deck is an epidemic card?
  2. For both distributions, what is the probability that two cards in a row are epidemic cards?
  3. For both distributions, what is the probability that three cards in a row are epidemic cards?
  4. For both distributions, what is the probability that all epidemic cards are in a row?
  5. What have we learned about how these distributions compare?
Ok, these are hard questions, particularly if you work through them in order 1 - 5. Without giving away the answers, I think it is striking that:
  1. the answer is the same for both distributions, but what follows is very different!
  2. left for reader
  3. much higher for modified set-up
  4. actually not too hard to calculate for each distribution; again, much higher for modified set-up
  5. very different for potential impact on game play
Conditional probabilities
I'm open to other suggestions, but currently think that the key way to see the difference in these two ways of shuffling is to focus on conditional probabilities. For method 1, say p1(n|i) is the probability that the nth card will be an epidemic card, given that there have been i epidemic cards already drawn up to that point and similar for p2(n|i). For a fixed i, even the domains of definition of p1 and p2 aren't the same!

What happened when we played?
There's one key fact you need to know to understand our actual game: J1 hasn't learned how to really shuffle yet. That's right, all the epidemic cards were in one cluster!

J1 asked me to include some other information about our game play:

  • J1 really likes to be the medic. At the start of the game, it is the most helpful piece
  • Dispatcher plus medic is a very powerful combination. The dispatcher becomes even more powerful than the medic once cures have been found.
  • We don't really follow the hand-limit rule.  This feels like an unnatural restriction that doesn't fit with the story of the game.

How a hacker gets started?

who: J1
when: early afternoon
where: online

J1 came up with a fun and surprising word game.

You know those promotions where you eat/buy something, then get a code to enter online for a game or drawing? Recently, one of J1's favorite snacks has been doing a treasure/pirate themed contest. Each "serving" comes with a code that, once entered, gives him 1-5 plays of a little game.
Pre-processed form of fav snack


Three interesting tidbits:
  1. We know there are 20 code words and we don't have all of them
  2. Only one valid code can be entered each day, but it seems an unlimited number of attempts can be made
  3. The codes are all related to treasure
What has this inspired? J1's first hacking attempts (to my knowledge).

Hacking + Pirates ≅ Win! (mod LittleBoy)

He has been thinking of treasure words and trying them in the system.  He was successful today and that's sure to encourage him for tomorrow.

Monday, January 5, 2015

Goody bags (fair sharing)

Who: J1
When: before bedtime
Where: bedroom

Several months ago, I wrote a post (here) about various sharing problems and the difficult question: "What is fair?" I got distracted until I found some ready-made examples from Peter Liljedahl. J1 and I discussed his Goody Bags scenario last night.

Though it seems simple or childish, I found this activity stimulated a rather deep conversation and was really effective. I highly recommend other parents try it.

In a nutshell, you have these 40 gifts to distribute among 5 friends and want to make the distribution fair:


J1 cut out all of the pictures, spent a couple minutes putting them in piles, then we arranged them and talked about whether the allocation was fair.  Each rough column below is a goody bag:

When discussing, we didn't start with them all in a grid, but worked a column at a time, starting from the right.  that means we had two goody bags out and compared them against each other.  J1 naturally came to the arrangement that put common items side-by-side and he was careful to leave spaces where one bag got something that was missing in the other.

As we discussed, he was pretty clear that the common items weren't relevant to deciding whether the two goody bags were fair, we only needed to look at the differences. When talking about comparing unlike objects, he tended to focus on how permanent they would be and compared the difference baskets rather than individual objects within them.  For example, the two rightmost bags differ by +tatoo+sticker - kitkat - gel pen. He judged this fair because the kitkat wouldn't last long, the gel pen was nearly permanent while enjoyment of the the tatoo and sticker would both be moderately lived.

At each step, we compared the new goody bag to the single preceeding goody bag that was most similar.  Eventually, J1 proclaimed that he was satisfied with the distribution and didn't really have a preference between any of the goody bags.

We talked about some of the characteristics of his division:

  • when there were 5 of some item, everyone got one
  • when there were more than 5 of some item, everyone got at least one
  • everyone got the same total number of items.
  • no one got more than 1 duplicate

Next, we talked about how he had divided the items. He explained that he had simply done it randomly, by which he meant that he took all the items and dealt them out in a circle. I asked whether it would work well if we tried it again, so he did, but made a very important change.  This time, he dealt the items in a line and gave two items to the bags on the ends in each pass. If that's hard to picture, call the bags A, B, C, D, E and then he is giving goodies to the bags in this order:
A- B - C - D - E - E - D - C - B - A - A - B - C - D - E - E - D - C - B - A (etc).

This process gave us a very different result (not pictured) where most of the previous characteristics were violated:

  • A and E got a lot of duplicates
  • Often, someone wouldn't get an item, even if there were 5 or more

Everyone still got the same total number, but it was hard to call this split "fair."

We had a good discussion about why the results were different. This included a really interesting idea: if the process used to divide the goodies was "fair," then maybe the ending distribution is fair even if it doesn't look right.  Essentially, he was defending randomness and arguing that everyone had an equal chance to get any particular goody, so the opportunity was fair. This was a pretty sophisticated idea that I hadn't expected to hear. I think our experiment of dealing goodies in a circle vs the line method also demonstrated that apparent randomness can be thwarted with subtle systematic biases, another rather deep idea.

We moved on to discuss whether he had any personal preferences among the items. He ranked them as below, most favored on the left and least on the right. Items that are stacked are considered equal and there is some sense of spacing.



Lastly, he revealed that he had 3 strategies for attacking this challenge, to be employed sequentially, if needed:
  1. deal cards randomly
  2. swap items if they seemed to be blatantly unfair
  3. give up on the whole project and just put all his favorites in one goody bag and day-dream that he got to keep it.
J1's closing comment
When we were all done, I told J1 I was glad we had finally done this sharing challenge. He looked up and said: "oh, this is what you meant! When you said we should do the sharing challenge the other day, I thought you were going to make us share our actual toys, that's why I didn't want to do it."

J2's observation
J2 and I talked a little about this investigation. His solution was simple:

  • first row goes to bag A
  • second row goes to bag B
  • third row goes to bag C
  • fourth row goes to bag D
  • fifth row goes to bag E
  • then the remaining three rows allocated to each bag (in order) by column
He declared this fair because everyone got the same number of items and was entirely unperturbed by the idea that, for example, someone would get 5 potato-head keychains. So, maybe the conversation won't be deep with every child. Or, this indicates stages of maturity in thinking about what fairness is?

Sunday, January 4, 2015

Odd one out

who: J2 and J1
when: after breakfast
what materials: Mental Maths 7-8, Test 21



J2: Daddy, what is the correct answer?
J0: What do you think?
J1: I think it is (a) because the circle is 2-dimensional while the cone and the cylinder are 3-d.
J0: What about you, J2?
J2: I think (b) because the cone is the only one with a sharp point.
J0: Hmm. What about the cylinder? is there a reason it could be the odd one out?
<a while for thining>
J1: The circle and the cone are both used to make a pseudosphere (see yesterday!) while the cylinder isn't, so the answer is (c)!
J0: Great idea! So what does it mean to answer this type of question?
.....

What is your reasoning?

We proceeded to talk about how any of the choices could be an odd one out and that a proper answer to this type of question was at least some identified property in which the odd-one differs from the other two.  I emphasized that the best thing you can do is try to come up with as many different ideas as possible and we talked about some:
  • number of dimensions
  • number of vertices, sides, faces
  • straight vs curved sides
  • number and types of symmetries
We also talked about ideas of what other people would be most likely to list. I don't know if this is important, but I'm often curious to see ways in which they stratify their mathematical thinking. For both of them, it was very clear that being useful for making a curve of constant negative curvature (not yet how they phrased it, ha!) was a more esoteric and sophisticated property.

Those Diagrams . . .

One other thing we could have talked about: what is actually being depicted in the three choices? For examples of what these diagrams could be:
a) Circle, disc, or solid shape with circular disc base viewed from a perspective that hides teh rest of the shape
b) Cone boundary (with or without the disc at the base), solid cone, or circular segment with another circular arc forming a loop
c) cylinder boundary (with or without discs on the end), solid cylinder, or rounded rectangle with extra curve arc forming a loop

The ANSWER

In the final segment of the discussion, we guessed what was on the answer key and J2 took a look. It simply said (a). I told them that, while I could accept a lot of answers, I considered that unacceptable, made a big "X" with my arms and a buzzer sound.