The least common multiple (LCM) of two numbers is the smallest number that is a multiple of both of those numbers. To find the LCM of 3 and 5, you can list the multiples of each number until you find a multiple that they have in common:
Multiples of 3: 3, 6, 9, 12, 15, 18, 21, 24, 27, 30
Multiples of 5: 5, 10, 15, 20, 25, 30
The least common multiple of 3 and 5 is 15, because it is the smallest multiple that both 3 and 5 have in common.