The factors of 12 are 12, 6, 4, 3, 2, 1.
The factors of 66 are 66, 33, 22, 11, 6, 3, 2, 1.
The common factors of 12 and 66 are 6, 3, 2, 1, intersecting the two sets above.
In the intersection factors of 12 ∩ factors of 66 the greatest element is 6.
Therefore, the greatest common factor of 12 and 66 is 6.
