On a 120 km track, a train travels the first 30 km at a uniform speed of 30 km/h. How fast
must the train travel the next 90 km so as to average 60 km/h for the entire trip?
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Total Distance = 120 km.
As, average speed = total distance ÷ total time taken
60 = 120 ÷ time taken
Time taken = 120 ÷ 60
= 2 hours
In 1st hour it travelled 30 km as its speed was 30 km/hr.
So, time taken by it to travel 90 km was 2 hr - 1 hr = 1 hr.
So its speed in 2nd case was = Distance ÷ Time taken
90 ÷ 1 = 90 km/ hr. ANS
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