What sort of process do you use for doing subtraction in your head? Like, if I had to figure out 52-37

Hil, I would do it kind of like you, only not entirely.

37 rounds to 40, and 52-40 is 12, then I add back in the 3 that took 37 to 40, for a total of 15.

I certainly was never taight to do math in my head, though I did get some tips on approximations at some point.

I wasn't ever formally taught it in school. I kind of worked out my own methods for it as a kid, though, since my parents would always have us do stuff like that. Like, if we went out to eat, my mom would hand the bill to one of us kids and ask us to check to make sure it was added up properly, or ask us to figure out the tip, or if we were at the grocery store and looking at a few different brands of something that came in different-sized boxes, she's ask us to figure out the cost per ounce to figure out which was the better deal. Never told us how to do the calculations, just trusted us to figure out a way to do it. (Which led to my sister and me both being the annoying aghast "You had to take out a calculator for that?" people as adults. But we could calculate 15% by the time we were 8 or 9.)

Oh, I do it Steph's way, too, sometimes. I am inconsistent. And probably would try three different approaches before I'd believe the answer, really.

Heh. I amazed a friend of mine (Classics major) by comparing the per ounce prices of something we were shopping for. Only I did it by reading the little labels on the store shelf, where it was helpfully printed. She thought it was Math Major Mojo.

37 rounds to 40, and 52-40 is 12, then I add back in the 3 that took 37 to 40, for a total of 15.

Huh. Neat.

Brains are nifty things. I remember reading a study that was done with some poor Brazilian kids. They'd all worked at their parents market stalls since they were pretty young. When they were given a sheet of standard subtraction problems, they didn't do too well at it. But when they were asked things like, "If I wanted to buy something that cost 37, and I gave you 200, how much change would you give me?" they were generally all able to do it with no problem, generally by using some method like "100 + 50 + 10 + 3."

Hil, I do math in my head much the same way.

Too bad more people don't.

Case in point - the time I was at Borders and, for some reason, the register didn't do the math. The cashier couldn't figure out how much change to give me. We spent a fair amount of time there with him trying to figure it out. sheesh.

I'm so pleased with myself. I finished the bit of work I needed to today already. So now I have the rest of the day to work on my Electronic Publishing paper. Yay?

I think I subtract in my head the way Hil does. But that might just be because I'm reading the example and agreeing that it makes sense.

I come in here all gronky, and you're talking arithmetic?

Duck tapes head together to keep it from exploding.

Last year at a dance some students were selling pizza for $1.25. I gave them a five, and one of my own students gave me $4.75 in change.

Fortunately it was one of my science students, so I didn't feel so personally responsible, but boy, did I give him a hard time over it.