At a height of 15 meters above the surface of a freshwater lake, a sound pulse is generated. The echo from the bottom of the lake returns to the point of origin 0.130 s later. The air and water temperatures are 20°C. How deep is the lake? I know that the velocity of air at 20 degrees Celsius is 343 m/s. I know that the velocity of freshwater at 20 degrees Celsius is 1482 m/s. ... How in the heck do I figure this out??