Speed of Sound

[mrross]

A stone is dropped into a well. The sound of the splash is heard 3.96 s later. What is the depth of the well?

 

[Danger]

Welcome to PF, Mrross.
This would appear to be a homework question, in which case it's in the wrong sub-forum. What have you done to try solving the problem? What is the speed of sound at STP? What is the atmospheric pressure and temperature in the well, if other than STP? Both make a difference to the speed of sound, and thus to the calculation.