Earlier quoted context omitted.
> I think the song is very funny and charming, but I do think this is a rare case where Lehrer is just wrong, the "borrowing" style is a much better and clearer way to explain subtraction. How much of that is that you're familiar with this method (the same way Tom was familiar with the old method)?
It's a good question, I'm not sure. I do think it's clearer what's going on, and the steps are more obvious. Like, there's a joke in the song about what to do with the carry, if you add it to subtrahend digit or remove it from the minuend digit ("if you're over 35 and went to public school...") which to me indicates that it's rather arbitrary and "learn algorithm by rote". Like, the "borrowing" thing just much better…
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[What if the digit in the subtrahend is bigger than the one in the minuend?]
Imagine adding 10 to both numbers. Obviously, the difference between them will not be changed.
But adding 10 to the digit in the subtrahend is the same as adding 1 to the digit immediately to its left.
So, add 10 to the [current] digit in the minuend, add 1 to the [next] digit in the subtrahend, and then perform the subtraction.