a
(a)
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 $=78.4 / 0.232=338\,m / s$