There are 600 grams of 6% salt water, how many grams of water need to be evaporated to get 20% salt water.
有6%的食盐水600克，要蒸发多少克水，就得到20%的食盐水．
<gadget id="calculator">6 / 100</gadget>
<output>3/50 = around 0.06</output>

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

<gadget id="calculator">(3/50) / (1/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">600 * (3/10)</gadget>
<output>180</output>

<gadget id="calculator">600 - 180</gadget>
<output>420</output>

<result>420</result>