40% of the girls in the calligraphy class are equal to 50% of the boys. There are 15 girls, how many boys are there?
书法班女生的40%与男生的50%相等，女生有15人，男生有多少人？
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(2/5) / (1/2)</gadget>
<output>4/5 = around 0.8</output>

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

<result>12</result>