After building a canal, after completing the full length (1/5), there is still 120 kilometers away from the midpoint. How many kilometers is the total length of this canal?
修一条水渠，完成了全长的(1/5)后，离中点还有120千米，这条水渠全长是多少千米？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/2) - (1/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">120 / (3/10)</gadget>
<output>400</output>

<result>400</result>