There are 42 people in class 31, 25 people participated in the composition competition, 30 people participated in the mathematics competition, each person participated in at least one kind of competition, how many people participated in both kinds of competitions.
三1班有42人，期中有25人参加了作文比赛，30人参加了数学竞赛，每人至少参加一种，两种比赛都参加的有多少人．
<gadget id="calculator">30 - 42</gadget>
<output>-12</output>

<gadget id="calculator">25 + (-12)</gadget>
<output>13</output>

<result>13</result>