The school has two interest groups. There are 42 people in class 3 and 1. Among them, 27 people participate in the calligraphy and painting group, and 24 people participate in the chess group. How many people participate in both groups?
学校开设两个兴趣小组，三1班有42人参加，其中27人参加书画小组，24人参加棋艺小组，两个小组都参加的有多少人
<gadget id="calculator">24 - 42</gadget>
<output>-18</output>

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

<result>9</result>