There are 21 girls in class 3 and 4, and the number of boys is twice that of girls. How many people are there in Class 3 and Class 4?
三4班有女生21人，男生是女生的2倍．三4班一共有多少人．
<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<gadget id="calculator">21 + 42</gadget>
<output>63</output>

<result>63</result>