Question: what is the greatest common factor in 9 and 11?
Answer: The factors of 11 are 11, 1. The factors of 9 are 9, 3, 1. The common factors of 11 and 9 are 1, intersecting the two sets above. In the intersection factors of 11 ∩ factors of 9 the greatest element is 1. Therefore, the greatest common factor of 11 and 9 is 1.