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Question

On a 120km/h track ,a train travels the first 30km at a uniform speed of 30km/h .how fast must the train travel the next 90 km somas to average 60km/h for the entire trip?


Solution

Given that,

Total distance covered, S = 120 km 

First 30 km is covered at 30 km/h. 

Time taken is, t 1 = 30/30 = 1 hour 

Let the train covers next 90 km at speed x km/h. 

Then the time taken is, t 2 = 90/x hour 

Average speed = total distance/total time 

=> 60 = 120/(t 1 + t 2 ) 

=> 60 = 120/(1 + 90/x) 

=> 1 + 90/x = 2 

=> 90/x = 1 

=> x = 90 km/h 

Thus, to achieve an average speed of 60 km/h the train must travel the next 90 km at 90 km/h. 


Physics

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