If you have a number multiplied by 9, take the number, put a zero behind it, and subtract the number from it.
For example, 9x3 --> 3 put zero to get 30, then subtract 3 to get 27
What you're basically telling the kid is to multiply by 10 and subtract the number after that. But if they were told they were "multiplying by 10" they'd get confused easier, hence putting a zero behind it will help them to multiply by 9.
For us, we're saying:
9*2 = (2*10)-2
Or
9x = (10x)-x
