There are 42 people in class 3 and 2, of which 27 people participate in the Chinese interest class, and 24 people participate in the English interest group. How many people participated in both.
三2班有42人，其中有27人参加语文兴趣班，有24人参加英语兴趣组．有多少人两项都参加了．
<gadget id="calculator">24 - 42</gadget>
<output>-18</output>

<gadget id="calculator">27 + (-18)</gadget>
<output>9</output>

<result>9</result>