h = 78.4 m

g = 9.81 m/s2

u = Initial speed = 0

Time taken to hear the sound = τ = 4.23 sec

a) Speed of helmet (v = ?)

Use kinematics engine (free fall)

v ^2 = u ^2 + 2gh

= 0 + 2gh

V=√2gh

V=39.22m/s

Time taken by helmet to reach the ground is t = ?

Use kinematics egh.

v = u + gt

t=39.22/9.81

hence t=3.99s

hence time difference is 4.23-3.99=0.24s

speed of sound in air is =78.4/0.24=326.67m/s

HOPE IT HELPS YOU