When digging a canal, the dug is 20% of the undug, and the dug is 400 meters less than the undug. How much is the undug?
挖一段水渠，已挖的是未挖的20%，已挖的比未挖的少400米，未挖的多少米？
<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">400 / (4/5)</gadget>
<output>500</output>

<result>500</result>