Question: how many numbers between 1 and 100 are divisible by 3 and 5?
Answer: There are 20 numbers divisible by 5 between 1 and 100, and 33 numbers divisible by 3 between 1 and 100. So there are 20 + 33 = 53, so 53 numbers divisible by one or the other, but this also includes every number which is divisible by both 5 and 3 twice.