Given any two numbers, which is greater, the lowest common multiple of the numbers or the greatest common multiple?

After you've calculated a least common multiple, always check to be sure your answer can be divided evenly by both numbers. Find the LCM of these sets of numbers. Multiply each factor the greatest number of times it occurs in any of the numbers. 9 has two 3's, and 21 has one 7, so we multiply 3 two times, and 7 once.