A TV set is sold at a 20% discount, which means how much the current price is lower than the original price.
一台电视机，打8折出售，就是现价比原价降低了多少．
<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>

<result>1/5 = around 0.2</result>