A train $S_{1}$, moving with a uniform velocity of $108 \mathrm{~km} / \mathrm{h}$, approaches anoth

Question

A train $S_{1}$, moving with a uniform velocity of $108 \mathrm{~km} / \mathrm{h}$, approaches another train $S_{2}$ standing on a platform. An observer $O$ moves with a uniform velocity of $36 \mathrm{~km} / \mathrm{h}$ towards $S_{2}$, as shown in figure. Both the trains are blowing whistles of same frequency 120 Hz . When $O$ is 600 m away from $S_{2}$ and distance between $S_{1}$ and $S_{2}$ is 800 m , the number of beats heard by $O$ is $\qquad$ .\ (Speed of the sound $=330 \mathrm{~m} / \mathrm{s}$ ). Report nearest integer\

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(8.128)\