[Math] How many 3 digit numbers have no zeros, how many have no zeros or fives…

combinatorics

How many 3-digit numbers have no zeros? How many 3-digit numbers have no zeros and no fives? How many 3-digit numbers have no zeros but at least one five?

I'm just really confused on how to do this. I figured the first was 729 and the second was 512 but I'm very stuck on the third one.

Best Answer

HINT

For the third case let start from case 1 and subtract the number of cases without 0 and 5.