If 800 grams of salt water with a concentration of 16% is to be concentrated to 20%, how many grams of salt should be added?
浓度为16%的盐水800克，要加浓到浓度20%，应加盐多少克．
<gadget id="calculator">16 / 100</gadget>
<output>4/25 = around 0.16</output>

<gadget id="calculator">1 - (4/25)</gadget>
<output>21/25 = around 0.84</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">(21/25) / (4/5)</gadget>
<output>21/20 = around 1.05</output>

<gadget id="calculator">800 * (21/20)</gadget>
<output>840</output>

<gadget id="calculator">840 - 800</gadget>
<output>40</output>

<result>40</result>