Question: what is the greatest common factor of 5 and 7?
Answer: The factors of 5 are 5, 1. The factors of 7 are 7, 1. The common factors of 5 and 7 are 1, intersecting the two sets above. In the intersection factors of 5 ∩ factors of 7 the greatest element is 1. Therefore, the greatest common factor of 5 and 7 is 1.