Find the smallest multiple of 15 such that each digit of the multiple is either 0 or 4

The answer is 4440. You can determine this by using your rules of divisibility. We know that is must end in a 0 or 5 to be divisible by 5 and that the sum of the digits must be divisible by 3 in order to divide by 3. One or two 4's together are not divisible by 3. Three 4's are, but it must end in 0 to be divisible by 5 as well.