If the width of a rectangle is reduced by 20%, but the area remains the same, then the length should be increased by what percentage.
若一个长方形的宽减少20%，而面积不变，则长应当增加百分之多少．
<gadget id="calculator">10 * 5</gadget>
<output>50</output>

<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">5 * (4/5)</gadget>
<output>4</output>

<gadget id="calculator">50 / 4</gadget>
<output>25/2 = around 12.5</output>

<gadget id="calculator">(25/2) - 10</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">(5/2) / 10</gadget>
<output>1/4 = around 0.25</output>

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