There are 600 grams of salt water with a concentration of 15%, how many grams of water need to be evaporated in order to make salt water with a concentration of 20%.
有浓度为15%的盐水600克，为了制成浓度为20%的盐水，需要蒸发掉多少克水．
<gadget id="calculator">15 / 100</gadget>
<output>3/20 = around 0.15</output>

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

<gadget id="calculator">(3/20) / (1/5)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">600 * (3/4)</gadget>
<output>450</output>

<gadget id="calculator">600 - 450</gadget>
<output>150</output>

<result>150</result>