There are 300 grams of salt water with a concentration of 20%, and to prepare 40% salt water, how many grams of salt water with a concentration of 70% should be added.
有浓度为20%的盐水300克，要配制成40%的盐水，需加入浓度为70%的盐水多少克．
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

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

<gadget id="calculator">(2/5) - (1/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">(7/10) - (2/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(1/5) / (3/10)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">300 * (2/3)</gadget>
<output>200</output>

<result>200</result>