How much water must be evaporated from 40 kg of 12.5% brine to produce 20% brine?
要从含盐12.5%的盐水40千克中蒸去多少水分才能制出含盐20%的盐水？
<gadget id="calculator">12.5 / 100</gadget>
<output>0.125</output>

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

<gadget id="calculator">0.125 / (1/5)</gadget>
<output>0.625</output>

<gadget id="calculator">40 * 0.625</gadget>
<output>25</output>

<gadget id="calculator">40 - 25</gadget>
<output>15</output>

<result>15</result>