5 x 3 = 15

[notrichenough]

The more things change…

Some of you who have small children may have perhaps been put in the embarrassing position of being unable to do your child’s arithmetic homework because of the current revolution in mathematics teaching known as the New Math.
…
But in the new approach, as you know, the important thing is to understand what you’re doing, rather than to get the right answer.
…
And so you’ve got thirteen tens, and you take away seven, and that leaves five…
Well, six actually… but the idea is the important thing!

• Tom Lehrer, 1965

https://www.youtube.com/watch?v=DfCJgC2zezw