There are 20 kg of brine with 15% salt content, how many kg of salt should be added to make the brine contain 20% salt.
有含盐15%的盐水20千克，要使盐水含盐20%，需要加盐多少千克．
<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">20 * (17/16)</gadget>
<output>85/4 = around 21.25</output>

<gadget id="calculator">(85/4) - 20</gadget>
<output>5/4 = around 1.25</output>

<result>5/4 = around 1.25</result>