Question

In a 120 km track a train travels the first 30 km/h . Calculate the speed with which the train move rest of the track so as to get the average speed of 60 km/hr for the entire trip

Answer 1

Answer:

Rest of the speed is 90 km/h

Explanation:

distance d1 = 30 km,      v1 = 30 kmph  

  => time taken = t1 = d1/v1 = 1 hour

So distance remaining = d2 = 120 - d1 = 90 km

let speed to cover distance d2 = v2  kmph

  => time taken = d2/v2 = 90/v2  hours

average speed = total distance / total duration of time

              60 kmph = 120 km / [1 hour + 90 / v2  hours]

      =>  1 = 2 / [ 1 + 90 / v2 ]

      =>  v2 = 90 kmph

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another way:

  as the total distance = 120 km

  average speed = 60 kmph  

      => total time duration = 120 / 60 = 2 hours

 

  time taken for the first 30 km =  30 km /30 kmph = 1 hours

  Hence, time remaining to cover the remaining 90 km = 2 hrs - 1 hrs = 1 hour

  => speed during 90 km  = 90 kmph

Answer 2

Answer:

90 km/h

Explanation:

t=d/v

t=120/30=4h

d=v*t

d=60*4=240

average speed = 120+240/4

=360/4=90(ans)