PROBLEM OF THE DAY

This problem is a physics problem, for those in an algebra 2 or higher should be able to work this out, with some thought. A person needs to know how deep a well is, but has no way to measure it since its to dark. He drops a large pebble into the well, and hears the sound of the pebble hit the water 5 seconds later. How deep is the well? also, is it possible to determine the speed of the pebble right before it hit the water? Do your best, and please leave some comments.