Question: what is a common factor between 6 and 30?
Answer: The factors of 30 are 30, 15, 10, 6, 5, 3, 2, 1. The common factors of 6 and 30 are 6, 3, 2, 1, intersecting the two sets above. In the intersection factors of 6 ∩ factors of 30 the greatest element is 6. Therefore, the greatest common factor of 6 and 30 is 6.