Question: what is the least common factor of 18 and 54?
Answer: The prime factorization of 18 is: 2 x 3 x 3.
The prime factorization of 54 is: 2 x 3 x 3 x 3.
Eliminate the duplicate factors of the two lists, then multiply them once with the remaining factors of the lists to get lcm(18,18) = 54.