As per doppler's Effect of sound we can say when subway is approaching the platform we will have
[tex]f_1 = f_o* \frac{v}{v- v_s}[/tex]
[tex]12750 = f_o* \frac{340}{340 - v_s}[/tex]
Similarly we can find the frequency when subway is passing away with same speed from us
[tex]f_2 = f_o* \frac{v}{v + v_s}[/tex]
[tex]10750 * f_o* \frac{340}{340 + v_s}[/tex]
now we can find the ratio of two
[tex]\frac{12750}{10750} = \frac{340 + v_s}{340 - v_s}[/tex]
[tex]1.186*(340 - v_s) = 340 + v_s[/tex]
[tex]403.26 - 1.186 v_s = 340 + v_s[/tex]
[tex]63.26 = 2.186 v_s[/tex]
[tex]v_s = \frac{63.26}{2.186}[/tex]
[tex]v_s = 28.94 m/s[/tex]
So the speed of subway is 28.94 m/s
