The teacher asked two math problems. Among 30 people, 21 people got the first problem right, and 18 people got the second problem right. How many people got both problems right?
老师出了两道数学题，在30人中，做对第一题的有21人，做对第二题的有18人，两道题都做对的最少有几人？
<gadget id="calculator">21 + 18</gadget>
<output>39</output>

<gadget id="calculator">39 - 30</gadget>
<output>9</output>

<result>9</result>