The distance between Delhi and Agra is 200 km. A train travels the first 100 km at a speed of 50 km/h. How fast must the train travel the next 100 km, so as to average 70 km/h for the whole journey?

Solution: 
 

Time Required To Maintain An Average Speed Of 70km/hr  = Distance/Speed 
=> 200/70
=> 20/7 hr

Now,
Time Taken For The First Case = Distance/Speed
=> 100/50
=> 2 hr

Now, Time Remaining
=> (20/7) - 2 
=> 6/7 hr

Speed = Distance/Time

Remaining Distance = 100 km

Speed = 100/(6/7)

Speed = 100 × 7/6
=> 700/6
=> 350/3 
=> 116.6 km/h

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