The numbers that have 15 as a multiple are 1, 3, 5 and 15. Becasue, 1*15 = 15; 3*5 = 15; 5 * 3 = 15; and 15*1 = 15. The easiest way to do that is to factor 15 in the prime factors: 15 = 1*3*5, and then see the possible combinations, which are 1 * 15, 3*5, 5 * 3 and 15 * 1. Therefore 15 is multiple of 1, 2, 3 and 15 itself.
