Well I don't know !
Let's figure it out:
-- The factors of 18 are 1, 2, 3, 6, 9, and 18 .
-- The factors of 30 are 1, 2, 3, 5, 6, 10, 15, and 30 .
-- The numbers that are factors of both 18 and 30 (the 'common' ones)
are 1, 2, 3, and 6 .
-- The biggest one on that short list (the 'greatest common' one) is 6 .
