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.

Depending on the size of the numbers involved, I either subtract like Hil does, or I round the biggest number up to the nearest 5 or 0, and add the number necessary to the smaller number, and then subtract. So 52 rounds up to 55, which took 3, so I add 3 to 37, which makes it equal 40, and then 55-40=15. I normally use the latter method with bigger numbers (3 digits or more) - makes the wrangling easier.

omg the maths.

I have eaten my lunch, and now have nothing else to do for 4.5 hours. The wait is ever the more excrutiating because I will be meeting EllenS for dinner tonight!

In mememe news, I bought a pair of really silly boots. Comfortable and will keep my toes warm (if it ever gets cold), but very silly. Also MUCH less expensive than Zappos lists them for.

My boots. Let me show you them. (I got them in black)

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?

For your example, I guess I go backwards and use addition to get to the next ten (37+3=40). I then add that to the 12 I see at that point. I realize there is subtraction to get to the 12 but I don't "see" it in my head that way. I see it as needing 12 to reach 52.

I've never really thought about it, but I know that in college I was able to do much of the addition/subtraction in accounting class in my head, when other people used a calculator.