In a class, boys account for 4/5 of the girls, and there are 15 girls. How many boys are there?
一个班里，男生占女生的(4/5)，女生是15人，男生多少人？
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">15 * (4/5)</gadget>
<output>12</output>

<result>12</result>