In a sports meeting, there are 15 people from Class A who participated in the field competition, 12 people who participated in the track competition, 7 people who participated in the field competition and track competition, and 21 people who did not participate in the competition. So how many people are there in Class A?
在一次运动会中，甲班参加田赛的有15人，参加径赛的有12人，参加田赛又参加径赛的有7人，没有参加比赛的有21人．那么甲班共有多少人．
<gadget id="calculator">12 - 7</gadget>
<output>5</output>

<gadget id="calculator">15 + 5</gadget>
<output>20</output>

<gadget id="calculator">20 + 21</gadget>
<output>41</output>

<result>41</result>