Question

On a 120 km track A train travels the first 30 km at a uniform speed of 30 km per hour calculate the speed with which the train should more rest of the track so as to get the average speed of 60 km for the entire trip

Answer 1

Speed of the train in the first 30 km, v1 = 30 km/h
Speed of the train in the next 90 km, v2 (to be determined)
Average speed = 60 km/h = (v1 + v2)/2 
                                          = (30 km/h + v2)/2 

⇒ v2 = 90 km/h

Answer 2

                           ANSWER                            

                          In the first 30 km speed of the train, v1 = 30 km/h

                          Speed of the train in the next 90 km, v2 (to be determined)

                         Average speed = 60 km/h = (v1 + v2)/2  

                                         = (30 km/h + v2)/2  

                              → v2 = 90 km/h