PLEASE PLEASE help, none of my classmates and friends can figure this out we are suffering T_T

There are two parts to this solution:  The down part for the helmet, and the up part for the sound.  The down part has to be solved first, before the up part.

-- The helmet falls 78.4 meters.  How long does that take ?

In gravity,

Distance = (1/2) (acceleration) (Time²)

78.4 m = (1/2) (9.8 m/s²) (Time²)

Time² = (78.4 m) / (4.9 m/s²)

Time² = 16 sec²

Time = 4 seconds.

-- The crew heard the sound of helmet hitting the ground 4.23 seconds after their buddy dropped it.

We just calculated that it took 4 seconds for it to hit the ground.

So it took 0.23 second for the sound of the clunk to propagate the 78.4 meters back up to them.  What's that speed ?

Speed = (distance) / (time)

Speed = (78.4 m) / (0.23 sec)

Speed = 340.9 m/s

inverse v=i*r

nice nice