Here: Initial velocity of stone $u=0$

Height of the tower $h = 500 \, m$
Height of tower is given by,

$h = ut + \frac{1}{2}gt^2$ $500 = 0 \times t + \frac{1}{2} \times 10 t^2$ $5 t^2 = 500$ $or \ t = 10 \ sec$

(where t is the time taken by the stone to reach water level in lake)
The time taken by the sound of splash to reach the point from where the stone is dropped is given by,

$= \frac{\text{height of tower}}{\text{velocity of sound}}$ $= \frac{500}{330} = 1.5 \text{ sec}$

Hence, the sound of splash will be

= 10 + 1.5 = 11.5 sec