How many kilograms of water are added to 20 kilograms of salt to obtain salt water with a salt content of 4%.
往20千克盐中加入多少千克水，可得到含盐率为4%的盐水．
<gadget id="calculator">4 / 100</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">20 / (1/25)</gadget>
<output>500</output>

<gadget id="calculator">500 - 20</gadget>
<output>480</output>

<result>480</result>