In order to get 18, we have the ratio 2:1. 2 represents the blue marbles and 1 represents the red marbles. We now have 12 blue marbles in a box. 2 x 6 = 12 so we are multiplying by 6. What we do to one number, we do to the other so 1 x 6 = 6 . Add 12 + 6 = 18
