Question

An observer is riding on a bicycle and moving towards a hill at $18 \,kmh ^{-1}$ He hears a sound from a source at some distance behind him directly as well as after its reflection from the hill If the original frequency of the sound as emitted by source is $640\, Hz$ and velocity of the sound in air is $320\, m / s$, the beat frequency between the two sounds heard by observer will be _______Hz

Answer 1

The correct answer is 20