There is 300g of salt water with a concentration of 20%, how many grams of salt is needed to turn it into salt water with a concentration of 40%?
现有浓度为20%的食盐水300g，要把它变成浓度为40%的食盐水，需要食盐多少克？
<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">300 * (4/3)</gadget>
<output>400</output>

<gadget id="calculator">400 - 300</gadget>
<output>100</output>

<result>100</result>