Thats very simple bro
       given height,s=500m
       speed of sound =340
 on analysing we get that  we have to find time period
first analyze the data and apply different formulas for it then see which formula suits the best.
         the formula that should be used here is
we get t=10 sec
 now the time for the resound=500/340=1.47 sec
 total time=10+1.47=11.47 sec
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S = 500 m = distance to travel - for the stone
g = 10 m/sec/sec,        initial velocity of stone  u = 0
equation of the dynamics of stone :
     s = u t + 1/2 g t^2
       500 = 0  + 1/2 * 10 * t^2

  t =  10 sec

Immediately after touching the water, in a fraction of second, there is a sound wave produced on the surface of water - splashing of water.

time for sound to reach the top = distance to travel / speed = 
                 500 meter / 340 m/sec  = 25/17 sec

total time = 10 sec + 25/17 sec

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