If 50g of salt water containing 40% salt is converted into salt water containing 20% salt, how many grams of water should be added?
把含盐40%的盐水50g，改制成含盐20%的盐水，需要加多少g的水．
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">50 * (2/5)</gadget>
<output>20</output>

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

<gadget id="calculator">20 / (1/5)</gadget>
<output>100</output>

<gadget id="calculator">100 - 50</gadget>
<output>50</output>

<result>50</result>