Question

To hear a clear echo, the reflecting surface must be at a minimum distance of:

• A. 10m
• B. 16.5m
• C. 33m
• D. 66m

Answer 1

The correct option is B 16.5m

Distance travelled by sound waves is $2d$.
To hear a clear echo, the time gap should be minimum $0.1$ between the two sounds.

So, minimum distance of reflecting surface $d=\frac{v\times t}{2}=\frac{330\times 0.1}{2}=16.5m$