Explanation:
The first echo travels from the helicopter to the surface and back. The second echo travels from the helicopter to the surface to the bottom of the lake and then back.
We know the difference in time is 0.24 s, so the time it takes the sound wave to travel through only the water is 0.24 s. Since the sound wave covers the distance twice (first from the surface to the bottom, then the bottom to the surface), we only want half the time, 0.12 s.
The speed of sound in water at 20°C is 1481 m/s. Therefore:
d = 1481 m/s × 0.12 s
d = 178 m
The depth of the lake is 178 meters (round as needed).
