Now there are 500 grams of salt water with a concentration of 3%, how many grams of water needs to be evaporated to make salt water with a concentration of 4%.
现在有浓度是3%的盐水500克，为了制成浓度是4%的盐水，需要蒸发掉水多少克．
<gadget id="calculator">3 / 100</gadget>
<output>3/100 = around 0.03</output>

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

<gadget id="calculator">(3/100) / (1/25)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">500 * (3/4)</gadget>
<output>375</output>

<gadget id="calculator">500 - 375</gadget>
<output>125</output>

<result>125</result>