How do you find the lowest common multiple

To determine the least common multiple of two numbers, determine the prime factors of both numbers. Then, determine the prime factors they have in common. Multiply the two given numbers together, and divide by the prime factors they have in common. (Side note: the product of these common prime factors is the two numbers' "greatest common factor.").

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 3s, and 21 has one 7, so we multiply 3 two times, and 7 once.