New Math Tom Lehrer

You can't take three from two
Two is less than three
So you look at the four in the tens place
Now that's really four tens
So you make it three tens
Regroup, and you change a ten to ten ones
And you add 'em to the two and get twelve
And you take away three, that's nine
Is that clear?

Now instead of four in the tens place
You've got three
Cause you added one
That is to say, ten, to the two
But you can't take seven from three
So you look in the hundreds place

From the three you then use one
To make ten ones
(And you know why four plus minus one
Plus ten is fourteen minus one?
Cause addition is commutative, right!)
And so you've got thirteen tens
And you take away seven
And that leaves five