A "Children's Encyclopedia" is sold at 20% off for 40 yuan, how much cheaper than the original?
一本《儿童百科全书》打八折出售是40元，比原来便宜了多少元？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">40 / (4/5)</gadget>
<output>50</output>

<gadget id="calculator">50 - 40</gadget>
<output>10</output>

<result>10</result>