Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed $1.0 m s ^{-1}$ and the man behind walks at a speed $2.0 ms ^{-1}$. A third man is standing at a height $12 m$ 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 $1430 Hz$. The speed of sound in air is $330 m s ^{-1}$. At the instant, when the moving men are $10 m$ apart, the stationary man is equidistant from them. The frequency of beats in $Hz$, heard by the stationary man at this instant, is. . . . . 
  • A$4$
  • B$5$
  • C$8$
  • D$10$
IIT 2018, Advanced
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