As long as how many grams of water are added to 250 grams of brine with a concentration of 40%, the concentration becomes 25%.
在250克浓度为40%的盐水中只要加入多少克水，浓度就了变成25%．
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

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

<gadget id="calculator">(2/5) / (1/4)</gadget>
<output>8/5 = around 1.6</output>

<gadget id="calculator">250 * (8/5)</gadget>
<output>400</output>

<gadget id="calculator">400 - 250</gadget>
<output>150</output>

<result>150</result>