what are the greatest common factors for 49 and 21?
['The factors of 21 are 21, 7, 3, 1.', 'The factors of 49 are 49, 7, 1.', 'The common factors of 21 and 49 are 7, 1, intersecting the two sets above.', 'In the intersection factors of 21 ∩ factors of 49 the greatest element is 7.', 'Therefore, the greatest common factor of 21 and 49 is 7.']