Xavier needs 84 paper plates for a housewarming party. He already has 21 green plates and 28 red plates. How many more plates should Xavier buy?
The total number of green and red plates is 28 + 21 = 
<gadget id="calculator">28+21</gadget>
<output>49</output>
49.
Xavier should buy 84 - 49 = 35 more plates.

<result>35</result>