Find the speed of sound in air?
speed = distance/time but you have to deduct the time for the stone to fall.
t = √(2d/g) =√(200/9.8) = 4.52 seconds
s = 100m/(4.81-4.52)s = 100m/0.29s = 344.8m/s
Speed varies according to temperature and humidity, so this answer is an average figure for that particular experiment. The speed of sound in dry air at 20 °C = 343 m/s.