problem How and which equation?? Please help me! At a height of 10 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 0.140 s later. The air and water temperatures are 20°C. How deep is the lake? equations d=vt v=wavelength*frequency wavelength = 4(L + 0.4d) Vsound = 331.5m/s + .6T ok heres what I got from a friend and I know its right but... Speed in air: 343 m/s Speed in water: 1482 m/s Time in air = 20/343 = 0.058 s Time in water = 0.140 - 0.058 = 0.082 s Time to hit bottom: 0.082 / 2 = 0.041 s d = 1482 (0.041) = 60.8 m ...where did the 20 come from on "Time in Air"? is it 10 x 2?