Question: what is the greatest common factor of six and four?
Answer: The factors of 6 are 6, 3, 2, 1. The factors of 4 are 4, 2, 1. The common factors of 6 and 4 are 2, 1, intersecting the two sets above. In the intersection factors of 6 ∩ factors of 4 the greatest element is 2. Therefore, the greatest common factor of 6 and 4 is 2.