A train S1, moving with a uniform velocity of 108 km/h, approaches another train S2 standing on a platform. An observer O moves with a uniform velocity of 36 km/h towards S2, as shown in figure. Both the trains are blowing whistles of the same frequency 120 Hz. When O is 600 m away from S2 and the distance between S1 and S2 is 800 m, the number of beats heard by O is ________

A train S1, moving with a uniform velocity of 108 km/h, approaches another train S2 standing on a platform. An observer O moves with a uniform velocity of 36 km/h towards S2, as shown in figure. Both the trains are blowing whistles of the same frequency 120 Hz. When O is 600 m away from S2 and the distance between S1 and S2 is 800 m, the number of beats heard by O is _________. (Speed of the sound = 330 m/s)

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