Question: what is the greatest common factor between 15 and 64?
Answer: The factors of 64 are 64, 32, 16, 8, 4, 2, 1. The common factors of 15 and 64 are 1, intersecting the two sets above. In the intersection factors of 15 ∩ factors of 64 the greatest element is 1. Therefore, the greatest common factor of 15 and 64 is 1.