Question: what is the common factor between 9 and 64?
Answer: The factors of 64 are 64, 32, 16, 8, 4, 2, 1. The common factors of 9 and 64 are 1, intersecting the two sets above. In the intersection factors of 9 ∩ factors of 64 the greatest element is 1. Therefore, the greatest common factor of 9 and 64 is 1.