Ok well, first of all, you must do 12 x 2 because if Mike has twice as many of red marbles, (12 red) then you must multiply that by 2. So 12 x 2 is 24! Then Mike has 3 times as many of yellow marbles, so you do 24 x 3, which is 72! So the answer is A, Sandy is correct. Hope this helps! :)
