Multiples with no large digits

Here’s a curious theorem I stumbled across recently [1]. Take an integer N which is not a multiple of 10. Then there is some multiple of N which only contains the digits 1, 2, 3, 4, and 5. For example, my business phone number 8324228646 has a couple 8s and a couple 6s. But 6312 × 8324228646 […]