A man standing 510m away from a wall sounds a whistle. The echo from the wall reaches him 3s later. Calculate the velocity of sound in air.

Analyze the question: A sound travels from the man to the wall and back again as an echo. The total time for this round trip is 3 s, and the man is 510 m from the wall.

Concept: For an echo, the sound covers twice the distance between the source and the reflecting surface.

Velocity = total distance ÷ time = 2d / t

Solution:

• Distance to the wall, d = 510 m
• Total distance travelled by the sound = 2 × 510 = 1020 m
• Time taken, t = 3 s

v = 1020 / 3 = 340 m/s

Checking against the options: 340 m/s is option B. It is also close to the well-known speed of sound in air at room temperature (about 330-340 m/s), so the result is sensible.

Common mistake: If you forget the round trip and compute 510 / 3, you get 170 m/s (option A). That is the trap option. Another wrong approach is multiplying 510 × 3 = 1530 (option E).

Exam tip: Whenever you see the word "echo", remember: v = 2d / t. Quickly check the answer against the known value of about 330-340 m/s to confirm you did not forget the doubling.