Li Mao bought a stock at the beginning of the year. The stock fell by 20% that year. How much should it rise in the second year to maintain its original value.
李毛年初买了一个股票，该股票当年下跌了20%，第二年应上涨多少才能保持原值．
<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">(1/5) / (4/5)</gadget>
<output>1/4 = around 0.25</output>

<result>1/4 = around 0.25</result>