At a height of 15 meters above the surface of a lake, a sound pulse is generated. The echo from the bottom of the lake returns to the point of origin 0.150 s later. The air and water temperatures are 20°C. How deep is the lake?

Answer 1

look up speed of sound in water at 20C
call it Cw
look up speed of sound in air at 20C
call it Ca
then distance in water = 2d
distance in air = 30
time in water = t
time in air = (.15 -t)
then
time in air = 30/Ca = .15 - t
solve for t
then
Cw t = 2 d