(i) It is due to the successive and multiple reflections of the sound from a number of reflecting surfaces, such as the clouds and the land.
The distance of the object from the submarine = 3625 m
Distance travelled by the signs = 2 x 3625 m
Time taken in sending out the signal and to receive back = 5 s
Speed = Distance/Time = (2 x 3625)/5
=1450 m s^-1