There are 40 kilograms of brine with 15% salt content, how many kilograms of salt need to be added to make the brine contain 20% salt?
有含盐15%的盐水40千克，要使盐水含盐20%，需要加入盐多少千克？
<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">40 * (17/16)</gadget>
<output>85/2 = around 42.5</output>

<gadget id="calculator">(85/2) - 40</gadget>
<output>5/2 = around 2.5</output>

<result>5/2 = around 2.5</result>