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A child’s non-commutative model of multiplication

December 3, 2013 am31 11:52 am

I was playing math with a niece and nephew a few weeks ago. Really, just playing games. And challenges.

We played who can get to 12 (by adding 1s or 2s). I wasn’t going for a rule, but my niece was close, so we (sister-in-law helped) got her to discover that 9 was a good number. And so was 3. And my nephew (younger) wasn’t going to discover it, but once his sister announced it, he kind of sort of followed.

We played puppies and kittens (game I learned from Sue Van Hattum. Adopt as many puppies as you like. Or as many kittens. Or an equal number of each. And – here’s a twist – whoever adopts the last furry animal loses). The two kids played with each other (I watched), and while the girl discovered some strategy, it was not a complete solution, and the two seemed to enjoy it.

I broke out some wonderful dice that my games mentor gifted me. Blue dice have the numbers 5 – 10 on the sides. Red dice have 0 – 5. 2cm, wood. I gave my niece one red, and I rolled one blue, and we saw who got higher. I won two or three rounds before she called me on it. Then I gave her two red dice, and him one blue one, and they rolled against each other, sum of the red against the blue. And then I gave her five red dice, and him 2 blue dice, and they both had some quick adding to do. They played for almost fifteen minutes, and needed to be stopped. Completely engaged. (And no, not a fair game. I didn’t calculate the probability, but the expected value favored the younger child. Intentionally, to help maintain interest).

I pulled out some graph paper (1/2 inch) and some crayons. Here I didn’t involve my nephew (I asked him to draw me something), but I drew a rectangle for my niece, 4×3, horizontally oriented. I counted the boxes (12) and the lines on the outside (14). I asked her if she could make another 12 box rectangle. She copied mine. I asked her if she could make a different 12 box rectangle. She drew a 2 unit high rectangle, and counted, and closed it at 6 wide. I asked her to draw another 12 box rectangle. She drew a 6×2, but this one vertically oriented. I asked her to count lines for each rectangle – 14, 14, and 16.

My sister-in-law asked my niece if she could write a multiplication for each rectangle. Not where I was headed. But I understand that it is not obvious to non-teachers that not every encounter with mathematics needs to reach “fruition.” And it was fine, the girl knows a little bit about multiplication, so I sat back, and watched.

Next to the 4×3 she wrote 4 x 3 = 12. Next to the next 4×3 she wrote 4 x 3 = 12. Next to the 6×2 she wrote 6 x 2 = 12. Next to the vertical 6×2 she wrote 6 x 2 = 12, and started to cross it out. My sister-in-law started to speak, to interrupt the process, but I motioned to let my niece continue, and she did. And after crossing out 6 x 2 = 12, my niece wrote 2 x 6 = 12.

I was delighted. My sister-in-law was concerned. She wanted her daughter to see that 4 x 3 and 3 x 4 were the same thing. I did not. I thought the girl was in a good place, was developing a strong sense of multiplication, and would transition nicely, later. So I intervened to assuage her mother’s concerns while only denting, not exploding, her non-commutative model. I turned the paper, and let her conclude that a 4 x 3 could be a 3 x 4 if you looked at it differently. And I asked if 3×4 had the same number of boxes as 4×3. She answered without pausing. And 2×6 and 6×2? Ditto. Right, 4×3 and 3×4 in her mind were different things, with the same answer, and that’s ok.

I dragged out the rectangles challenges by asking if there was a different rectangle with 12 boxes with even more lines. She was stuck, so I drew a 1×12. She carefully counted. 26 boxes. I asked if that was the most, she was not sure, I began to draw 1/2 by something, counting half boxes with her along the way (she was good at counting by halves!), and she was certain that there were more lines. She counted anyhow. My brother, who had only watched part of this, asked if we would ever be done (with the most lines) and she articulated nicely a “keep cutting in half” approach.

Then I taught them Set (or rather, what makes a set. I turn teaching someone how to play Set into an enjoyable game itself. I’ve done this with high school and middle school students for years. The first day we never play). And then I sent them some turn-taking rules that I thought would be better for adults playing with little kids, and kids of different ages playing together. (I played these rules on Thanksgiving, 2 math teachers and a 2nd grader, fun for all).

I never wrote about going to the Math Circle 2013 summer conference a few months ago, at Notre Dame. But I believe my experiences there had some influence on this story.


10 Comments leave one →
  1. December 3, 2013 pm31 8:28 pm 8:28 pm


  2. Christopher permalink
    December 4, 2013 pm31 5:41 pm 5:41 pm

    If you don’t mind my asking, how old are the niece and nephew?

    • December 4, 2013 pm31 6:00 pm 6:00 pm

      Not at all. My niece is in 2nd grade, my nephew just started kindergarten.
      This is not the level I usually engage with (high school, intro level or pre-intro level college, and occasional forays into middle school), so, while I had fun, I would appreciate critical commentary, negative and positive. Thanks.

      • Christopher permalink
        December 4, 2013 pm31 6:10 pm 6:10 pm

        Thanks! This is a lovely account. I have recommended it to others. Thanks for writing it up!

  3. December 5, 2013 am31 8:25 am 8:25 am

    Could you tell us the modified Set rules for taking turns?

    • December 5, 2013 pm31 4:21 pm 4:21 pm

      I wrote them out in some detail, but the short version is, deal out 12, player A takes their turn (hunts for a set. You can choose to set a time limit). If they find one, they explain why it is a set, and its Player B’s turn.
      If A doesn’t find one, the other players get to hunt (you can choose to limit the time). If someone finds one, they get it, and play resumes with player B.
      If A doesn’t find one, and no one else does, deal out 3 more cards and A goes again.

      Play goes in a circle until there are no more sets.

      This puts a premium on accuracy, while deëmphasizing speed, which is a good thing for young beginners, and will make them better players later.

    • December 5, 2013 pm31 4:22 pm 4:22 pm

      Another reduced game would be to knock out all the empties and stripes, leaving a short deck of 27. Much easier to find a set this way.

      I am not sure though, that this promotes better play for the full game. The jump from 3 attributes to 4 might be huge.

      • December 26, 2013 am31 12:20 am 12:20 am

        This (one third deck) is how I learned Set (in 2nd grade, from my math teacher). I believe his rule was that if several people find a set simultaneously, the quietest one to call it gets to claim it. I have also played with mixed crowds (but all of at least high school age) that experienced players may only take sets if they find two sets simultaneously.


  1. Multiplication and rectangles | Talking Math with Your Kids
  2. Where my niece got her non-commutative model of multiplication | JD2718

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