For a commodity, at the current price, the profit is 20% of the cost. If the cost is reduced by 20%, the profit is still at the current price.
一件商品，按现在的价格，利润是成本的20%，若成本降价20%，仍按现在的价格，利润是成本的多少．
<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 * (4/5)</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>