A train S1, moving with a uniform velocity of $108 \mathrm{~km} / \mathrm{h}$, approaches another train S2 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 $\_\_\_\_$ .
[Speed of the sound $=330 \mathrm{~m} / \mathrm{s}$ ]