To distill water from 40 kg of water with a salt content of 16% to produce brine with a salt content of 20%, how much water should be evaporated?
要从含盐16%的40千克水中蒸去水份，制成含盐20%的盐水，应蒸去多少水份？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">40 * (1/5)</gadget>
<output>8</output>

<gadget id="calculator">16 / 100</gadget>
<output>4/25 = around 0.16</output>

<gadget id="calculator">40 * (4/25)</gadget>
<output>32/5 = around 6.4</output>

<gadget id="calculator">8 - (32/5)</gadget>
<output>8/5 = around 1.6</output>

<gadget id="calculator">(8/5) / (1/5)</gadget>
<output>8</output>

<result>8</result>