Given 100 grams of salt water with a concentration of 10%, how many grams of salt should be added to obtain salt water with a concentration of 20%?
现有浓度为10%的盐水100克，想得到浓度为20%的盐水，需加盐多少克？
<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</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">(9/10) / (4/5)</gadget>
<output>9/8 = around 1.125</output>

<gadget id="calculator">100 * (9/8)</gadget>
<output>225/2 = around 112.5</output>

<gadget id="calculator">(225/2) - 100</gadget>
<output>25/2 = around 12.5</output>

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