Question: what is a common factor between 18 and 63?
Answer: The factors of 63 are 63, 21, 9, 7, 3, 1. The common factors of 18 and 63 are 9, 3, 1, intersecting the two sets above. In the intersection factors of 18 ∩ factors of 63 the greatest element is 9. Therefore, the greatest common factor of 18 and 63 is 9.