There are 48 students in a class, 15 of them take part in the math group, 18 take part in the model group, and 18 don't take part in either group, so how many people participate in both groups?
某班有48名学生，其中有15人参加数学小组，18人参加航模小组，有18人两个小组都不参加，那么两个小组都参加的有多少人．
<gadget id="calculator">15 + 18</gadget>
<output>33</output>

<gadget id="calculator">48 - 18</gadget>
<output>30</output>

<gadget id="calculator">33 - 30</gadget>
<output>3</output>

<result>3</result>