There is 20kg of liquid medicine with a concentration of 20%, how much water needs to be added to dilute it into a liquid medicine with a concentration of 10%?
现有浓度20%的药水20kg，要把它稀释成浓度为10%的药水，需要加水多少kg？
<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">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">4 / (1/10)</gadget>
<output>40</output>

<gadget id="calculator">40 - 20</gadget>
<output>20</output>

<result>20</result>