Q.An echo-sounder in a fishing boat receives an echo from a shoal of fish 0.45 s after it was sent. If the speed of sound in water is 1500 m/s, how deep is the shoal?

An echo is the repetition of an original sound, heard again after a short delay, caused by that sound reflecting off a rigid surface at some distance from its source -- sound obeys the same laws of reflection as light. Because the human brain retains ('hears') a sound sensation for roughly 0.1 second after it actually stops, a listener perceives a REFLECTED sound as a genuinely distinct, separate echo only if it returns at least 0.1 s after leaving the source.

At 22°C (speed of sound ≈344 m/s), sound covers $344\times0.1=34.4$ m in 0.1 s -- but since this is a ROUND TRIP (source to surface and back), the reflecting surface itself must be at least HALF that distance away, about 17.2 m, for a distinct echo to be heard. Because the speed of sound depends on air temperature, this minimum distance shifts somewhat as temperature changes. If the reflecting surface is instead closer than roughly 15 m, the returning sound merges with the original rather than forming a separate echo -- becoming reverberation instead.