The ratio of the boys in a class to the whole class is 6:13, of which there are 21 girls, how many boys are there?
一班男生与全班人数的比是6：13，其中女生有21人，男生有多少人．
<gadget id="calculator">13 - 6</gadget>
<output>7</output>

<gadget id="calculator">21 / 7</gadget>
<output>3</output>

<gadget id="calculator">3 * 6</gadget>
<output>18</output>

<result>18</result>