CBSE Class 9 Answered

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)

                                  = 68.5km/hr

regards

toppers expert