What is the greatest common factor of 96 and 108?

First reduce each number to its prime factors:

96 = 2 x 2 x 2 x 2 x 2 x 3

108 = 2 x 2 x 3 x 3 x 3

select only the numbers that appear twice in each list (ignoring multiple instances of the same number)

Thus the greatest common factor is 2 x 2 x 3 = 12