A number is divisible by 5 if the digit in the units or ones place is either 0 or 5. We want all digits to be even, so we throw out 5 as a candidate digit.
We're considering all numbers between 1000 and 9999, of which there are 9000. Numbers in this range can have any of the digits 0-9 in the tens and hundreds place, and any of 1-9 in the thousands place.
We also want all digits to be even, which means we can pull from the numbers {0, 2, 4, 6, 8} in the hundreds and tens places, and {2, 4, 6, 8} in the thousands place.
So there are 4*5*5*1 = 100 numbers in this range that are divisible by 5.
