A ambulance travels down a highway at a constant speed of…

  A ambulance travels down a highway at a constant speed of 55 mph, it siren emitting sound at a frequency of 500 Hz.  Ahead of the ambulance, a truck is traveling in the same direction at a speed of 75 mph.  What frequency does truck driver hear? (Note:  1 mph = 0.447 m/s and vsound = 343 m/s.  Select closest answer)