Question: what is a common factor between 15 and 18?
Answer: The factors of 18 are 18, 9, 6, 3, 2, 1. The common factors of 15 and 18 are 3, 1, intersecting the two sets above. In the intersection factors of 15 ∩ factors of 18 the greatest element is 3. Therefore, the greatest common factor of 15 and 18 is 3.