Question: what is the greatest common factor of 15 and 64?
Answer: The factors of 15 are 15, 5, 3, 1. The common factors of 64 and 15 are 1, intersecting the two sets above. In the intersection factors of 64 ∩ factors of 15 the greatest element is 1. Therefore, the greatest common factor of 64 and 15 is 1.