A motorcyclist is approaching a large wall with a velocity of $90\,km/hr$. A car is chasing him with a velocity of $108\,km/hr$. If the car sounds a horn at $30\,Hz,$ beat frequency heard by motorcyclist is ____ $Hz$ . Take velocity of sound $= 330\,m/s$ .
  • A$8.55$
  • B$7.05$
  • C$5$
  • D$3.45$
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