East of the Sun, West of the Moon

2007/10/11

Math

Filed under: Family — Erwin @ 1:04 am

When you do a subtraction like 13 – 7, you probably don’t even think about it anymore, you just spit out the answer, right? And just to be sure, that should be 6.

A surprised me this evening with a trick that he apparently figured out by himself. Instead of trying to do 13 – 7 in his head, he first subtracts 7 from 10, getting 3, and then adds that to 3, with final result being 6 (one hopes). So more generally speaking he uses the transformation (10 + x) – y == (10 – y) + x to turn a trickier subtraction into an easy subtraction and an addition!

I think that’s pretty cool for a fourth grader (a nine year old), but maybe I’m biased? 🙂

3 Comments

  1. that is, um, how I still do math at times 😛 spitting stuff out automatically *still* isn’t automatic for me. Well, something comes out. I just don’t trust it 🙂

    Comment by Leah — 2007/10/11 @ 3:11 am

  2. My mother actually taught me that trick. She also taught me that x + 9 == x-1 + 10, and x – 9 == x+1 – 10. So even still, when I get hit with “13 + 9” I sometimes do “13-1=12 + 10 = 22” in my head.

    -Kalle.

    Comment by Kalle — 2007/10/11 @ 6:29 am

  3. Yeah, I use those two as well. What I also do, if I feel unsure about my calculation, is 13 – 7 = 13 – (3 + 4) = 13 – 3 – 4 = 10 – 4 = 6. I tried to explain that to A but he didn’t want to pay attention for long enough for me to try. Oh well. Obviously he’s learned or figured out other ways to help himself with this. 🙂

    Comment by Erwin — 2007/10/11 @ 11:38 am

RSS feed for comments on this post.

Sorry, the comment form is closed at this time.

Powered by WordPress