Doppler effect and beats

JEE Advanced 2019 Paper 1, Question 15

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 same frequency 120 Hz. When \mathrm{O} is 600 m away from S2 and distance between \mathrm{S} 1 and \mathrm{S} 2 is 800 m, what is the number of beats heard by O?

[Speed of the sound =330 m/s ]

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When there is relative motion between the source and observer, the observed frequency f_o is …

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