To build a road, 20% of the total length was repaired in the first month, 300km in the second month, and 100km remained unrepaired. What is the total length of this road?
修一条公路，第一个月修了全长的20%，第二月修了300km，还剩下100km没有修．这条公路全长是多少？
<gadget id="calculator">300 + 100</gadget>
<output>400</output>

<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>