The distance between Delhi and Agra is 200 Km. A train travels the first 100 Km at a speed of 5 K/h . How fast must the train travel the next 100 Km , so as to average 70 K/h for the whole journey ?

Answer 1

To average at a speed of 70 km/h for the entire 200 km would take
200/70 hrs or appr 2.86 hrs

but going only 5 km/h for the first 100 km already took
100/5 hrs or 20 hrs. so it not possible to average 70 km/h for the whole trip

This is a common "trick" question that has been around in various forms
for a long time.
Usually the first leg is given with more reasonable data
e.g. doing the first 100 km at 25 km/h would have been a more reasonable situation
but led to the same conclusion.