Question

On a 120 km track a train travels the first 30 km at a uniform speed of 30 km/h. Calculate the speed with train should move rest of the track so as to get the average speed of 60 km/h for the entire trip.

Answer 1

Given that,

Total distance covered, S = 120 km

First 30 km is covered at 30 km/h. 

Time taken is, t1 = 30/30 = 1 hour 

Let the train covers next 90 km at speed x km/h.
Then the time taken is, t2 = 90/x hour

Average speed = total distance/total time 

=> 60 = 120/(t1+t2)
 60 = 120/(1 + 90/x)
 1 + 90/x = 2 
 90/x = 1
=> x = 90 km/h

Thus, to achieve an average speed of 60 km/h the train must travel the next 90 km at 90 km/

Answer 2

if the train travelled 30km/hour to complete 30 km so its speed should be 90km/hour to complete 90km.