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 
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.
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.