Answer :
Factors of 15 . . . 1, 3, 5, 15
Factors of 30 . . . 1, 2, 3, 5, 6, 10, 15, 30
Common factors (numbers on both lists) . . . 1, 3, 5, 15
Greatest common factor (biggest number on both lists) . . . 15
Factors of 30 . . . 1, 2, 3, 5, 6, 10, 15, 30
Common factors (numbers on both lists) . . . 1, 3, 5, 15
Greatest common factor (biggest number on both lists) . . . 15