There are 20 students in the third grade, 11 students like to play basketball, 15 people like to play table tennis, how many people like both.
三年级有20人，喜欢打篮球的有11人，喜欢打乒乓球的有15人，两样都喜欢的有多少人．
<gadget id="calculator">15 - 20</gadget>
<output>-5</output>

<gadget id="calculator">11 + (-5)</gadget>
<output>6</output>

<result>6</result>