The number of boys in Class 51 is 20% of the whole class, and the number of boys is equal to the number of girls
五一班男生人数是全班人数的20%，男生人数是女生人数的多少
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

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

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

<result>1/4 = around 0.25</result>