A stone is dropped from the
top of a tower 500m high into a
pond of water at the base of the
tower
When is the splash heard at the top?
Given, g = 10m/ssquare and speed of
sound = 340 m/s1

11.47 seconds is the answer.

given height,s=500m
g=10
speed of sound =340
upon analyzing 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
s=ut+1/2at2
u=0,
500=0+1/2x10xtxt
we get t=10 sec
now the time for the resound=500/340=1.47 sec
total time=10+1.47=11.47 sec

hope this helped!!