Inspired by the recent “Tom Lehrer Question” thread:

I’ve always wondered what the joke is on this line in the song “The New Math”:

Now remember how we used to do that. three from two is nine; carry the one, and if you’re under 35 or went to a private school you say seven from three is six,
but if you’re over 35 and went to a public school you say eight from four is
six; carry the one so we have 169…

I went to public school in the early 1960s; does that have something to do with why I don’t get it?

He’s doing the problem 342-173. Ok, so you understand the first step, 3 from 2 is 9, carry the 1.

Now, you have to carry that 1 somewhere. The older method is, you add 1 on the bottom…so even though the numbers are 4 and 7, you add the 1 to the 7, so you’re really doing 4-8. Around World War II, the way they taught that changed, so you took the 1 from the top, so the 4 became 3.

Does that make sense? It’s hard to explain using words

One of the features of New Math was that it tried to provide algorithms for subtraction that were mathematically rigorous, rather than those that were easier and just worked. (This was back in the days when people used pen and paper to subtract one number from another, rather than use a calculator or computer). But I never learned New Math – I learned the old-fashioned way, and was always reasonably good at it, so I’m not sure of the exact differences.

In this case, it’s two different old-fashioned ways of “carrying the one”. In the private school method, you carry the one by subtracting it from subtracting from the four to give three; in the public school method you carry the one by adding to the seven to give eight. But the two methods given the same result for that stage – six, and carry one.

the public school method mentioned is the Australian method also used in Europe. my parents who schooled in the 30s in the USA never heard of that method.

Said it several times before on this board, but the mentions are spread out over years: I learned Set Theory in the fifth grade and didn’t use it again until I took a class in college about databases. New Math was, to an extent, poorly planned out.