To build a canal, 30% of the total length was built on the first day, and 50% of the total length was repaired on the second day, leaving 400 meters to be repaired. How long is this canal in meters?
要修一条水渠，第一天修了全长的30%，第二天修了全长的50%，还剩400米没有修．这条水渠全长多少米？
<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (1/2) - (3/10)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">400 / (1/5)</gadget>
<output>2_000</output>

<result>2_000</result>