On a 120km track a train travel the first 30km at a uniform speed of 30km/hr calculate the speed with which the train should move rest of the track so as to get the average speed of 60km/hr for the entire trip
time taken to go 30 km at 30 km/h = 1 hour
time needed to go the entire 120 km at 60 km/h = 2 hours
so he has 1 hour to go the remaining 90 miles
rate = distance/time = 90 km/1 hr = 90 km/h