Question: what is a common factor between 18 and 21?
Answer: The factors of 18 are 18, 9, 6, 3, 2, 1. The factors of 21 are 21, 7, 3, 1. The common factors of 18 and 21 are 3, 1, intersecting the two sets above. In the intersection factors of 18 ∩ factors of 21 the greatest element is 3.