How many five-digit positive integers, divisible by 9, can be written using only the digits 3 and 6?
Well, to start, first go over the divisibility rule of 9: add up all digits and if the sum is divisible by nine, then the number is divisible by nine.
for ex: 3+3+3+6+3
four 3s and only one 6.
you can arrange four 3s and one 6 five ways: