A bus moving away from a wall with a speed of 10 m/s blows siren at frequency 680 Hz. If speed of sound in air is 330 m/s, the frequency heard by driver of bus is

freqency of the reflected sound with respect to bus driver is  (v-v0v+vo) n   observer and virtual source moving away from each other (330-10330+10) 680 =640 so he will hear both direct 680 and relected sound 640 Hz