Question: what is the greatest common factor for 7 and 9?
Answer: The factors of 7 are 7, 1. The factors of 9 are 9, 3, 1. The common factors of 7 and 9 are 1, intersecting the two sets above. In the intersection factors of 7 ∩ factors of 9 the greatest element is 1. Therefore, the greatest common factor of 7 and 9 is 1.