Question: what is the least common factor of 4 and 40?
Answer: The prime factorization of 4 is: 2 x 2.
The prime factorization of 40 is: 2 x 2 x 2 x 5.
Eliminate the duplicate factors of the two lists, then multiply them once with the remaining factors of the lists to get lcm(4,4) = 40.