There is 200 grams of a cup of salt water with a concentration of 15%, how many grams of salt should be added to make the concentration of this cup of salt water 20%?
有一杯浓度为15%的盐水200克，要使这杯盐水的浓度变为20%，请问要加入多少克盐？
<gadget id="calculator">15 / 100</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">1 - (3/20)</gadget>
<output>17/20 = around 0.85</output>

<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">(17/20) / (4/5)</gadget>
<output>17/16 = around 1.0625</output>

<gadget id="calculator">200 * (17/16)</gadget>
<output>425/2 = around 212.5</output>

<gadget id="calculator">(425/2) - 200</gadget>
<output>25/2 = around 12.5</output>

<result>25/2 = around 12.5</result>