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