How many kilograms of water needs to be evaporated when the farmer's uncle condenses 20 kilograms of 20% potion into 25% potion.
农民伯伯把20千克浓度为20%的药水浓缩成浓度为25%的药水，需要蒸发多少千克水．
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/5) / (1/4)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">20 * (4/5)</gadget>
<output>16</output>

<gadget id="calculator">20 - 16</gadget>
<output>4</output>

<result>4</result>