Eric has a total of 12 + 6 = 18 white and blue marbles. Thus, he has 20 - 18 = 2 green marbles.
This justifies what answer for what question? Q & A:
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?
2