A certain amount of water must be evaporated from 40 kg of brine containing 15% salt to obtain brine containing 20% salt. How many kilograms of water should evaporate?
要从含盐15%的40千克的盐水中蒸发一定的水分，得到含盐20%的盐水．应当蒸发掉多少千克水？
<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">15 / 100</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">40 * (3/20)</gadget>
<output>6</output>

<gadget id="calculator">8 - 6</gadget>
<output>2</output>

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

<result>10</result>