400 grams of salt water containing 5% salt is diluted with water to make salt water containing 4% salt, how many grams of water should be added?
含盐5%的盐水400克，加水稀释成含盐4%的盐水，需加水多少克．
<gadget id="calculator">5 / 100</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">4 / 100</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">(1/20) / (1/25)</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">400 * (5/4)</gadget>
<output>500</output>

<gadget id="calculator">500 - 400</gadget>
<output>100</output>

<result>100</result>