Naughty Number Nine

Number nine will put you on the spot
Number nine will tie you up in a knot
When you tryin' multiplying 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 multiplying 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 eighty minus eight
And seven times nine is seventy minus seven
And six times nine is sixty minus six
You could use those tricks
'Cause you must have some secret way you can beat it
Or else you'll need it
With naughty number nine

Of course, it doesn't hurt to know the table of nines by memory
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 digits' sum is always equal to nine. I mean, if you add two and seven, the digits, you get nine, the digits' 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 ol' number nine