Question: what is the greatest common factor for 30 and 90?
Answer: The common factors of 30 and 90 are 30, 15, 10, 6, 5, 3, 2, 1, intersecting the two sets above. In the intersection factors of 30 ∩ factors of 90 the greatest element is 30. Therefore, the greatest common factor of 30 and 90 is 30.