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

explanation: magnetic pole, region at each end of a magnet where the external magnetic field is strongest. the south-seeking pole, or any pole similar to it, is called a south magnetic pole. unlike poles of different magnets attract each other; like poles repel each other.

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.