The equation for the Doppler shift of a sound wave of speed v reaching a moving detector is

fd = fs [(v + vd) / (v-vs)]

where vd is the speed of the detector, vs is the speed of the source, fs is the frequency of the source, fd is the frequency of the detector. If the detector moves toward the source, vd is positive; if the source moves toward the detector, vs is positive.

A train moving toward a detector at 31 m/s blows a 305-Hz horn. What frequency is detected by
a. a stationary train?
b. a train moving toward the first train at 21 m/s?