A person in a vehicle is moving away from a vertical mountain with uniform velocity. When the vehicle is at a distance 975 m from mountain, the person blows a horn and hears its echo after 6 s. If velocity of sound in air is 340 ms$^{-1}$, the velocity of the vehicle is :

Let v be the velocity of the vehicle in m/s; since it moves away from the mountain, the sound travels to the mountain and then catches the moving vehicle. The sound's total path equals twice the initial distance plus the vehicle's displacement over 6 s: 2(975) + 6v. Using the formula distance equals speed multiplied by time gives the equation 1950 + 6v = 340(6). Solving 6v = 2040 - 1950 yields v = 15 m/s, which converts to 54 kmph.

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