Rajasthan Police Constable Model Paper 5

Let the length of train be L m
Speed = distance/time
Distance covered by train in 120 sec = (600 + L)
Speed = (600 + L)/120(1)
Distance covered by train in 80 sec = (300 + L)
Speed = (300 + L)/80(2)
Equating equation (1) and (2)
(600 + L)/120 = (300 + L)/80
⇒ 1200 + 2L = 900 + 3L
⇒ L = 1200 - 900 = 300 m