Question

on a 120km track, a train travels the first 30km at a uniform speed of 30 km/hr. how fast must the train travel the next 90km so as to average 60km/hr for the entire trip

Answer 1

V1 = 30km /hr 


Avg velocity = V1 + V2 / 2 

60 =  30 + V2 / 2 

120 - 30 = V2 

V2 = 90 Km /hr 

Answer 2

Solution :-

Total distance = 120 km
Initial distance covered = 30 km
Speed by which initial distance covered = 30 km/hr
Therefore time taken to cover the distance of 30 km = 1 hr
Average speed needed for the entire trip = 60 km/hr
Total time to be taken to cover 120 km = 120/60 = 2 hours
Remaining distance to be covered = 120 - 30 = 90 km
Time to be taken to cover the remaining distance of 90 km = 2 hr - 1 hr
= 1 hour
Therefore, speed needed for the remaining distance of 90 km = 90 km/hr

Hence, the train must travel the next 90 km at the speed of 90 km/hr so as to have an average of 60 km/hr for the entire journey.

Answer.