The least common multiple (LCM) is:
15 = 3 * 5
51 = 3 * 17
The LCM is like a parts list with enough parts to make 15 or 51. In this case the parts list is 3 * 5 * 17 because that has enough parts to make 15 or 51.
3 * 5 * 17 = 255
Answer: The LCM is 255.