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 = 
<gadget id="calculator">12+6</gadget>
<output>18</output>
18 white and blue marbles.
Thus, he has 20 - 18 = 
<gadget id="calculator">20-18</gadget>
<output>2</output>
2 green marbles.

<result>2</result>