A cup of 100 grams of salt water with a salt content of 4%, when how many grams of water are evaporated, the salt content of the salt water becomes 5%.
一杯含盐率为4%的100克的盐水，当蒸发掉多少克水后，盐水的含盐率变为5%．
<gadget id="calculator">4 / 100</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">100 * (1/25)</gadget>
<output>4</output>

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

<gadget id="calculator">4 / (1/20)</gadget>
<output>80</output>

<gadget id="calculator">100 - 80</gadget>
<output>20</output>

<result>20</result>