Fill a cup with 100 grams of sugar water, of which sugar accounts for 20%. After adding more grams of water to the cup, sugar accounts for 5% of the sugar water.
杯里装100克糖水，其中糖占20%，向杯里再加入多少克水后，这时糖占糖水的5%．
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">5 / 100</gadget>
<output>1/20 = around 0.05</output>

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

<gadget id="calculator">100 * 4</gadget>
<output>400</output>

<gadget id="calculator">400 - 100</gadget>
<output>300</output>

<result>300</result>