A product is priced at 400 yuan after a 20% discount. How much is this product cheaper than the original price?
一件商品打八折后售价400元，这件商品比原价便宜了多少元．
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">400 / (4/5)</gadget>
<output>500</output>

<gadget id="calculator">500 - 400</gadget>
<output>100</output>

<result>100</result>