posted by Ali .
5kg rock is dropped into a well and a splash is heard after 3 seconds , how deep is the well? Ignore air resistance
you need to know the speed of sound (v). The total time is time to fall plus time to wait for the sound.
If the depth is s,
s = 1/2 gt^2 + v(3-t)
Plug in g and v and you get s.
The 5kg is spurious. Time to fall does not depend on mass.