Question: what is the greatest common factor in 7 and 63?
Answer: The factors of 63 are 63, 21, 9, 7, 3, 1. The common factors of 7 and 63 are 7, 1, intersecting the two sets above. In the intersection factors of 7 ∩ factors of 63 the greatest element is 7. Therefore, the greatest common factor of 7 and 63 is 7.