Oscillations and Waves: JEE Main 2022 Physics question

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