The original price of a commodity is 80 yuan, now it is 20% off, how much is it cheaper?
一种商品原价80元，现在打八折，便宜了多少元．
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">80 * (1/5)</gadget>
<output>16</output>

<result>16</result>