Doppler Shift question

[Format]

A train moving toward a detector at 31m/s blows a 305-Hz horn. What frequency is detected by a statiory train?

Using the equation f´= f(V + Vd/ V - Vs)

The answer is supposed to be 340-Hz but I don't understand how to get it. Mind helping me out?

[Format]

anyone know?

[outandbeyond2004]

I do not recognize the formula. My scientific encyclopedia gives this for the source approaching the detector:

f' = f(\frac{1}{1-\frac{v}{v_s}})

where v_s is the speed of sound in air.