In 40 kg of salt water with a concentration of 16%, how many kilograms of water can be evaporated to increase the concentration to 20%.
在浓度为16%的40千克盐水中，蒸发多少千克水后可将浓度提高到20%．
<gadget id="calculator">16 / 100</gadget>
<output>4/25 = around 0.16</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(4/25) / (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">40 * (4/5)</gadget>
<output>32</output>

<gadget id="calculator">40 - 32</gadget>
<output>8</output>

<result>8</result>