List all the common multiples of 3 and 7 that are smaller than 40. Give the answer(s) in ascending order. (Eg 1, 2, 3)
Multiples of 3: 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39
Multiples of 7: 7, 14, 21, 28, 35
Common multiples: 21
Answer(s): 21