The distance between Delhi and Agra is 200 km.A train travels 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?

First determine the total time for the trip. To cover 200 Km at 70 Km/h will take:

time = distance/rate = \frac {200\ Km} {70\ Km/h} = 2.86 hours.

Next figure out how much time was spent on the first leg:

time of leg 1 = \frac {100\ Km} {50 \ Km/h} = 2 hours.

So that means the train must travel the second leg of 100 Km in 0.86 hours. Can you now determine what the answer is?

100/0.86 km/h

Thank you for the answer