Question: what is a greatest common factor of 9 and 39?
Answer: The factors of 9 are 9, 3, 1. The factors of 39 are 39, 13, 3, 1. The common factors of 9 and 39 are 3, 1, intersecting the two sets above. In the intersection factors of 9 ∩ factors of 39 the greatest element is 3. Therefore, the greatest common factor of 9 and 39 is 3.