There are equal numbers of boys and girls in a class. Among the students who like badminton (1/5) are boys, and girls who like badminton account for (1/4) of the whole class. It is known that there are 21 boys who don’t like badminton. Ask: How many girls like badminton?
某班男、女生人数相等，在喜欢羽毛球的同学中(1/5)是男生，喜欢羽毛球的女生占全班人数的(1/4)，已知不喜欢羽毛球的男生有21人，问：喜欢羽毛球的女生有多少人？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(1/5) / (4/5)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) * (1/4)</gadget>
<output>1/16 = around 0.0625</output>

<gadget id="calculator">(1/2) - (1/16)</gadget>
<output>7/16 = around 0.4375</output>

<gadget id="calculator">21 / (7/16)</gadget>
<output>48</output>

<gadget id="calculator">48 * (1/4)</gadget>
<output>12</output>

<result>12</result>