Question: what is the greatest common factor in 7 and 42?
Answer: The factors of 42 are 42, 21, 14, 7, 6, 3, 2, 1. The common factors of 7 and 42 are 7, 1, intersecting the two sets above. In the intersection factors of 7 ∩ factors of 42 the greatest element is 7. Therefore, the greatest common factor of 7 and 42 is 7.