"Naughty Number Nine" is a song in Schoolhouse Rock!, which can be seen in Multiplication Rock. The song teaches us how to multiply by nine.
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, eight times nine is 80 minus eight,
And seven times nine is 70 minus seven,
And six times nine is 60 minus six.
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:
One times nine is nine,
And two times nine is 18.
(Mean ol' number nine)
Three times nine is 27,
And four times nine is 36.
Five times nine is 45,
And six times nine is 54,
And seven times nine is 63.
Eight times nine is 72,
And nine times nine is 81,
And ten times nine is 90.
Now, the digit sum is always equal to nine.
I mean, if you add two and seven, the digits,
You'd get nine, the digit sum.
That's true of any product of nine.
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.