The original price of each pen is a yuan, what is the price after the price is reduced by 20%.
每枝钢笔的原价是a元，降价20%后的价格是多少．
<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>

<result>4/5 = around 0.8</result>