Question

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

Answer 1

Time taken to travel first half distance = 100/ 50
                                                             = 2 hrs Time taken to travel second half distance = 100/v ∴average speed = 200/ (2 + 100/v)⇒70 = 200/ (2 + 100/v) ⇒2 + 100/v = 20/7⇒100/v = 20/7 - 2 = 6/7⇒v = 100 * 7/6 Km/h ⇒v = 700/6 km/h