Let s be the distance between station a and b.
Let the time taken to travel distance s with the speed of 60km/hr = t1 = (s/60)hr
Let the time taken to travel distance s on return trip with the speed of 80km/hr = t2 = (s/80)hr
Now the total distance covered = 2s
so total time to complete the journey = t1+ t2 = ( s/60 + s/80 ) hr = s( 1/60+1/80)hr = 7s/240
So average speed = total distance / total time
= 2s / (7s/240)
Since the initial and final position of the train is same ,so zero displacement .
Thus average velocity will be 0 km/hr
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