Assuming no air (or air drag) on stone, the time it takes stone to reach water is:
\(t =\sqrt{2 h / g} =\sqrt{2(78.4) / 9.81}=\sqrt{15.98369}=3.998\,s\)
time for sound to travel \(78.4\,m =4.23-3.998=0.232\,s\)
speed of sound in well \(=78.4 / 0.232=338\,m / s\)