To build a road, 40% of the total length was built in the first half of the month, and 800 meters more in the second half of the month than the first half, and it was just finished. How long is this road in meters?
修一条公路，上半月修了全长的40%，下半月比上半月多修了800米，正好修完．这条公路全长多少米？
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5) - (2/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">800 / (1/5)</gadget>
<output>4_000</output>

<result>4_000</result>