Q. 39

The distance between Delhi and Agra is 400 km. A train travels the first 200 km at a speed of 100 km/h. How fast must the train travel the next 200 km, so as to average 70 km/h for the whole journey?

Answer :

total distance =400 km
average speed     = 70 km/h
time taken    = 400/70=40/7 h

1)During the first 200 km

speed= 100 km/h 
time taken t1 =200/100=2 h

2)During the second 200 km

t+ t2 = 40/7 

2 + t2  = 40/7
t2= 26/7 h

speed = distance / time= 200×7/26=1400/26 =53.84 km/h 
The speed of train during second 200 Km is 53.84 km/h to maintain an average speed of 70 km/h 

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