Echo distance from round-trip sound travel

Steps:

• Sound travels to cliff and back, total distance = 2 × ship-to-cliff distance (d).
• Total time for echo = 4.0 s.
• Speed of sound v = 320 m/s.
• Total distance = v × time = 320 m/s × 4.0 s = 1280 m.
• Ship-to-cliff distance d = 1280 m / 2 = 640 m.

Why B is correct:

• Echo time equals round-trip time, so d = (v × t) / 2 = (320 × 4) / 2 = 640 m, per wave reflection principle.

Why the others are wrong:

• A: Assumes one-way time, ignores return trip.
• C: Gives full round-trip distance, not to cliff.
• D: Doubles round-trip distance unnecessarily.

Final answer: B