A sound source of frequency 400 Hz is moving towards an observer with a velocity of 50 m/s while the observer is moving away from the source with a velocity of 40 m/s . Find the frequency heard by the observer . (Velocity of sound is 340 m/s )

Question

$(a)\;423.7\;Hz\qquad(b)\;391.6\;Hz\qquad(c)\;413.8\;Hz\qquad(d)\;405.7\;Hz$