JEE Main 2022 — Waves and Sound Question with Solution

An observer is riding on a bicycle and moving towards a hill at $18 \mathrm{km} {\mathrm{h}}^{-1}$. 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 $640 \mathrm{Hz}$ and velocity of the sound in air is $320 \mathrm{m} {\mathrm{s}}^{-1}$, the beat frequency between the two sounds heard by observer will be _____ $\mathrm{Hz}$.