JEE Physics Practice Question

Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed $1.0 \mathrm{~ms}^{-1}$ and the man behind walks at a speed $2.0 \mathrm{~m} \mathrm{~s}^{-1}$. A third man is standing at a height $12 \mathrm{~m}$ above the same horizontal line such that all three men are in a vertical plane. The two walking men are blowing identical whistles which emit a sound of frequency $1430 \mathrm{~Hz}$. The speed of sound in air is $330 \mathrm{~m} \mathrm{~s}^{-1}$. At the instant, when the moving men are $10 \mathrm{~m}$ apart, the stationary man is equidistant from them. What is the frequency of beats in $\mathrm{Hz}$, heard by the stationary man at this instant?