Naughty Number Nine: School House Rock video about multiplying by Nine.

Lyrics

Number Nine will put you on the spot.
Number Nine will tie you up, oh, in a knot.
When you're tryin'
Multiplyin' by nine,
You might give it everything you've got
And still be stopped.
If you don't know some secret way you
can check on,
You'll break your neck on
Naughty Number Nine.

Now the first thing to keep in mind
When you're multiplyin' by nine
Is that it's one less than ten.
You see nine is the same as ten minus one.
So you could multiply your number by ten,
And then subtract the number from the result,
And you'd get the same product
As if you'd multiplied by nine
And you knew it.
I mean 8 x 9 is 80 minus 8,
And 7 x 9 is 70 minus 7, and 6 x 9 is 60 minus 6.
You could use those tricks.
'Cause you must have some secret way you can beat it,
Or else you'll meet it
With Naughty Number Nine.

Of course it doesn't hurt
To know the table of nines by memory.
It goes like this:
1 x 9 is 9, and 2 x 9 is 18.
(Mean Old Number Nine)
3 x 9 is 27, and 4 x 9 is 36,
5 x 9 is 45, and 6 x 9 is 54, and 7 x 9 is 63,
8 x 9 is 72, and 9 x 9 is 81,
And 10 x 9 is 90

Now the digit sum is always equal to nine.
I mean, if you add 2 and 7, the digits,
You get 9, the digit sum.
That's true of any product of 9.
If they don't add up, you've made a mistake.
'Cause you must have some secret way you can check it,
Or else you'll wreck it
With naughty, nasty, mean old Number Nine.