What is the greatest common factor of 9 and 18?

To find the greatest common factor for any group of numbers you must make a list of all the factors for each number.

The factors of 18 are: 1, 2, 3, 6, 9, 18

The factors of 9 are: 1, 3, 9

Therefore, the greatest common factor of 18 and 9 is 9.