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? 
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
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
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