A certain township wants to build a water canal around the mountain. 50% of the total length has been repaired in the first phase, and 30% of the total length has been repaired in the second phase. There are still 800 meters left to be repaired. How many meters is this canal around the mountain?
某乡要修一条环山水渠，第一期修了全长的50%，第二期修了全长的30%，还剩800米没修．这条环山水渠长多少米？
<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">800 / (1/5)</gadget>
<output>4_000</output>

<result>4_000</result>