Question

on a 120 km track a train travells the first 40 km at 40 km/h speed . how much fast should the train travel the next 80 km so average speed is 60 km/h for the entire trip

Answer 1

s=d/t
40=40/t
t=d/t=40/40=1 (1)
Vavg=total d/total t
60=120/t
 t=120/60  
t=2hr
As it took 1 hr to travel 40km( refer 1)
it shall take (2-1)hr to travel 80km
s=d/t
s=80/(2-1)
s=80/1=80 kmph.
therefore answer is 80 km/h
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Answer 2

He travels the first 40 km at 40kmph
So he takes 1 hr to complete the first 40 km
Let the time taken to complete the remaining 80 km be 'x' hrs
We know that
Average Speed =
Total Distance Travelled / Total Time Taken
Thus....
60 = 120 / x + 1
60x + 60 = 120
60x = 60
Therefore x = 1
Thus he takes 1 hour to complete the remaining 80 km
Therefore he must travel the remaining 80 km with a speed of 80 kmph..
Hope this helps buddy!!