There are 20 students in class 31 who can play table tennis, 15 students who can play tennis, and 8 students who can play both kinds of balls. How many people are there in class 31?
三1班同学会打乒乓球的有20名同学，会打网球的有15名同学，两种球都会打的有8人，三1班共有多少人．
<gadget id="calculator">15 - 8</gadget>
<output>7</output>

<gadget id="calculator">20 + 7</gadget>
<output>27</output>

<result>27</result>