If 400 grams of salt water containing 40% salt is converted into salt water containing 20% salt, how many grams of water should be added?
把含盐40%的盐水400克，改制成含盐20%的盐水，需要加水多少克．
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

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

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

<gadget id="calculator">400 * 2</gadget>
<output>800</output>

<gadget id="calculator">800 - 400</gadget>
<output>400</output>

<result>400</result>