Question: which of these is a common factor of 18 and 42?
Answer: The gcf of 18 and 42 can be obtained like this: The factors of 18 are 18, 9, 6, 3, 2, 1. The factors of 42 are 42, 21, 14, 7, 6, 3, 2, 1. The common factors of 18 and 42 are 6, 3, 2, 1, intersecting the two sets above.