A train $\mathrm{S}1,$ moving with a uniform velocity of $108\mathrm{ }\mathrm{k}\mathrm{m}/\mathrm{h},$ approaches another train $\mathrm{S}2$ standing on a platform. An observer $\mathrm{O}$ moves with a uniform velocity of $36\mathrm{ }\mathrm{k}\mathrm{m}/\mathrm{h}$ towards $\mathrm{S}2,$ as shown in figure. Both the trains are blowing whistles of same frequency $120\mathrm{ }\mathrm{H}\mathrm{z}.$ When $\mathrm{O}$ is $600\mathrm{ }\mathrm{m}$ away from $\mathrm{S}2$ and distance between $\mathrm{S}1$ and $\mathrm{S}2$ is $800\mathrm{ }\mathrm{m},$ the number of beats heard by $\mathrm{O}$ is____
[Speed of the sound $=330\mathrm{ }\mathrm{m}/\mathrm{s}]$

Hence, the correct answer is 8.128.