A train travels 40 km at a uniform speed of 30kmh−1. Its average speed for the whole journey after travelling another 40 km is 45kmh−1. Its speed in the second half of the journey is ____.
90kmh−1
In the first half, distance covered = 40 km and speed = 30kmh−1,
then time taken to travel =distancespeed=4030 hours.
Let the speed of the train in later half be xms−1, then the time taken to travel later 40 km =distancespeed=40x hours
∴Total time taken =4030+40x
Total distance travelled = 40 + 40 = 80 km
Given,average speed = 45
Average speed =Total distanceTotal time=45
⇒ 804030+40x=45
⇒4x+120=8045×3x
⇒ x=90 kmh−1