To dilute 500 grams of a certain disinfectant with a concentration of 40% into a 5% disinfectant, how many grams of water should be added?
要将浓度为40%的某种消毒液500克稀释成5%的消毒液，需加水多少克？
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">5 / 100</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(2/5) / (1/20)</gadget>
<output>8</output>

<gadget id="calculator">500 * 8</gadget>
<output>4_000</output>

<gadget id="calculator">4_000 - 500</gadget>
<output>3_500</output>

<result>3_500</result>