Question: what is the greatest common factor gcf of 36 and 9?
Answer: The factors of 36 are 36, 18, 12, 9, 6, 4, 3, 2, 1. The common factors of 9 and 36 are 9, 3, 1, intersecting the two sets above. In the intersection factors of 9 ∩ factors of 36 the greatest element is 9. Therefore, the greatest common factor of 9 and 36 is 9.