I get it now - teachers are so effin' lazy not to be teaching the concepts that are the foundation of mathematics. + - * /
I think they’re trying to get the answer by adding up the difference between 12 and 32 in incremental steps. So the starting number is 12, and they add 3 to get 15. Then they add 5 to 15, to get 20. Then they add 10 to 20 to get 30. Then they add 2 to 30, to get 32, the ending number. Then they add all the individual results (3+5+10+2) to get 20. It seems very arbitrary.
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I agree that this is a stupid approach to teaching subtraction. But mentally, for simple problems it is the way I do subtraction in my head. Nobody taught me that and it is not at all useful for subtracting 145.266 from 376.22 for example. At least for me it isn’t.
It’s a cute trick for simple problems. Worth a couple of days of class time to supplement learning to subtract the old fashioned way when you don’t have a calculator/cellphone/iPad in your pocket.