Question: what is a factor of 11 and 55?
Answer: The factors of 55 are 55, 11, 5, 1. The common factors of 11 and 55 are 11, 1, intersecting the two sets above. In the intersection factors of 11 ∩ factors of 55 the greatest element is 11. Therefore, the greatest common factor of 11 and 55 is 11.