How many kilograms of pure salt need to be added to change 20 kilograms of brine with a salt content of 15% into brine with a salt content of 20%?
要将含盐为15%的盐水20千克，变为含盐20%的盐水，需要加入纯盐多少千克？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">20 * (1/5)</gadget>
<output>4</output>

<gadget id="calculator">15 / 100</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">20 * (3/20)</gadget>
<output>3</output>

<gadget id="calculator">4 - 3</gadget>
<output>1</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 / (4/5)</gadget>
<output>5/4 = around 1.25</output>

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