math test. Among the 36 students in the class, 21 people got the first smart question right, and 18 people got the second smart question right, and each of them got at least one question right. How many people are right about both?
一次数学测验．全班36人中，做对第一道聪明题的有21人，做对第二道聪明题的有18人，每人至少做对一道题．问两道都做对的有几人？
<gadget id="calculator">18 - 36</gadget>
<output>-18</output>

<gadget id="calculator">21 + (-18)</gadget>
<output>3</output>

<result>3</result>