The ratio of the number of boys to the number of girls in Class 5 is 6:7. There are 39 students in the class. How many girls are there?
五班男生人数与女生人数的比是6：7，全班有39人，女生有多少人？
<gadget id="calculator">6 + 7</gadget>
<output>13</output>

<gadget id="calculator">39 / 13</gadget>
<output>3</output>

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

<result>21</result>