When building a canal, when it reaches the full length (2/5), there is still 30 meters away from the midpoint. How many meters is the total length of this canal?
修一条水渠，当修到全长的(2/5)处时，离中点还有30米，这条水渠全长有多少米？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(1/2) - (2/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">30 / (1/10)</gadget>
<output>300</output>

<result>300</result>