Distance covered in 120 second = 1000 + length of train(l)
Distance covered in 80 seconds = 600 + l
So, distance covered in 40 seconds = (1000 + l) – (600 + l)
= 400 m
Speed = 400/40 = 10 m/s
Distance covered in 80 second = 80 x 10 = 800 m
So, 600 + l = 800
Length of the train (l) = 200 m