Question: what is the highest common factor of 11 and 55?
Answer: The factors of 11 are 11, 1. 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.