I think I do, 50 - 30 is 20, 7>2 so 10, 12 - 7 is 5, 15. Which may not be the best way to do it, since I've been known to skip the "7>2 so subtract 10" step, but it's what I do.

Hil, you'd probably be interested in this LJ entry (actually, both these entries), which poses almost the exact same question:

[link] [link]

She's got a tag for other math-related entries and math teacher stuff.

Hil, I think I do it the way you do it. I also add the same way - tens first then the ones.

Thanks Dana. That's really interesting.

Random question: What sort of process do you use for doing subtraction in your head? Like, if I had to figure out 52-37, I would think 52-30=22, 22-7=15, but I've been told that this seems strange to other people. I don't think I was ever taught that method -- it's just what seems natural to my brain. So do other people do it that way, or some other way, or think through the regular written process, with "borrow a ten, 2 becomes 12, 5 becomes 4" and so on?

I would do 52-7=45 and 45-30=15. No one taught me it either.

{{Suzi}} Glad you got a validating e-mail, and glad that there's a job with more money that you're qualified for, even if it is in SJ.

I subtract in my head much like you do, Hil, but it is laborious for me. I can do it but my fingers itch for a pencil the whole tinme and I don't really trust the answer unless I run through it a couple more times to be sure. Or count on my fingers. I certainly was never taight to do math in my head, though I did get some tips on approximations at some point.

I did have a college professor who taught tricks for multiplying largish numbers in your head, but I did not pay attention. It was a parlor trick sort of thing.

What sort of process do you use for doing subtraction in your head?

I count by tens from the lower number to the higher number, then figure the difference of the ones column. I can't visualize the numbers well enough to work it as a problem.

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.