For a commodity, the price is reduced by 20% for the first time, and then the price is increased by 20%. The current price is the price before the price reduction.
一种商品，第一次降价20%，接着又涨价20%．现价是降价前价钱的多少．
<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 + (1/5)</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">1 * (4/5) * (6/5)</gadget>
<output>24/25 = around 0.96</output>

<result>24/25 = around 0.96</result>