A 10 digit number is made up of only 5s and 0s. It's also divisible by 9. How many possibilities are there for the number?
Working backwards, I cam e to the conclusion that it cannot end in a 0, as that would been there was a remainder of 9 from the previous number. Then if it ends in a 5, that means a remainder of 4 was there from the previous. Now that could mean the 2nd last digit is either 0 or 5. But I'm lost after that, don't think I can apply this reasoning till the first digit.