Eric has 20 marbles. He has 12 white marbles, 6 blue marbles, and the rest are green marbles. How many green marbles does Eric have?
Eric has a total of 12 + 6 = <T>add(12, 6)=18 white and blue marbles.
Thus, he has 20 - 18 = <T>subtract(20, 18)=2 green marbles.
