Get the LCM (Least Common Multiple) of 3, 5 & 6
Multiples of
3: 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60
5: 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60
6: 6, 12, 18, 24, 30, 36, 42, 48, 54, 60
30 and 60 are the first 2 common multiples of the 3 numbers. but the least common multiple is 30.
So, the answer is 30.