As train is moving towards the stationary observer on the hill. And the wind is in direction of the motion of train, the frequency of the sound waves from horn heard to the observer on hill is given by n' = n [v'/v' - vs]
where v’ = v+w (sum of velocities of train and train). When this sound wave reflects from the hill, and travels towards the moving train, the frequency heard by the
driver is n' = n [v - w + vs/(v - w]
(a) The apparent frequency is given by
n' = n [v + w/(v + w)vs]
V = 1200 km/hr., w = 40 km/hr., vs = 40 km/hr. and n = 580 Hz.
n' = [1200 + 40/1200+40)-40] = 599.3Hz
(b) As shown in the figure, let the driver hear the echo when he is at a distance x km from the hill. Time taken by the train to reach the point B′