If 2 kg of salt water with 20% salt content is prepared to make salt water with 4% salt content, how many kg of water should be added?
含盐20%的盐水2千克，要配制成含盐4%的盐水，需要加入清水多少千克？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">4 / 100</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">(1/5) / (1/25)</gadget>
<output>5</output>

<gadget id="calculator">2 * 5</gadget>
<output>10</output>

<gadget id="calculator">10 - 2</gadget>
<output>8</output>

<result>8</result>