Question: what is a greatest common factor of 18 and 17?
Answer: The factors of 17 are 17, 1. The factors of 18 are 18, 9, 6, 3, 2, 1. The common factors of 17 and 18 are 1, intersecting the two sets above. In the intersection factors of 17 ∩ factors of 18 the greatest element is 1. Therefore, the greatest common factor of 17 and 18 is 1.