There are 600 grams of salt water with a concentration of 20%, how many grams of salt should be added to increase the concentration to 40%.
有浓度为20%的盐水600克，要把浓度增加到40%，应加盐多少克．
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">(4/5) / (3/5)</gadget>
<output>4/3 = around 1.333333</output>

<gadget id="calculator">600 * (4/3)</gadget>
<output>800</output>

<gadget id="calculator">800 - 600</gadget>
<output>200</output>

<result>200</result>