How many 4 digit numbers are such that the first digit is non-zero and it is overall a multiple of 5?

The idea for this is simply counting the number of possibilities of each digit. Since the number is a multiple of 5, then we must have it finishing with 0 or 5, i.e., we have 2 possibilities for the last digit. We have no constraints on the second and third digit, so we have 10 (0 can also be a digit) possibilities for these. Lastly, for the first digit, we must have it being non-zero, so it must be a number from 1 to 9, which gives us 9 possibilities. Hence, the number of such numbers is 21010*9=1800