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.

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 do this too. My mommy taught me in seventh grade math class.

Toddson--those boots are totally adorable.

Cute boots! I like the silly. They look warm, too, which is very appealing this morning.

Cute boots! I like the silly. They look warm, too, which is very appealing this morning

Seriously! My toes are little icicles (because I didn't have the sense not to wear sandals today).

Hah! I'd totally forgotten I'd gotten RIDIC x INFINITY purple boots for winter this year until I looked in my closet the other day.

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

Depending on the numbers, I'd either go with 52-30 = 22; 22-7 = 15 or else round the 37 up to be 40, as Tep said.

We actively teach both these (and various other) strategies to kids in primary school, with a lot of emphasis on the fact that different people will favour different strategies, and that so long as your method is a sound one, it's all cool - that there isn't One True Way of working out a problem. We do a lot of setting them problems and then focusing on HOW they got the answer, rather than what the answer was. it helps both in terms of picking up handy ways of doing things from other kids, and also in terms of identifying where you're getting mixed up, if you've got the wrong answer.

I mention this only because I don't think that I was taught in quite this way myself - I have more of an impression of there being impenetrable methods and little encouragement of discussion or risk-taking. (But my memory isn't neccesarily to be trusted, and is most subjective.)