How can use use the digits 6 and 4 to decide if 64 is the product of 9 and 7?

Answer 1

So there is a rule about multiples of 9 in general: something is a multiple of 9 if and only if its digits add up to a multiple of 9. 6+4 = 10, so it is not a multiple of 9. There's also a neat trick to remembering the multiples of 9 below 99 with your fingers, but you can look this up if you're interested.