[填空题]
Two automobiles are equipped with the samw82in,nq - hwme single frequency h a7(dbqccs 0r3orn. When one is at rest and the other is moving toward the firscqa0 b(dcrs73 t at 15 m/s the driver at rest hears a beat frequency of 5.5 Hz. What is the frequency the horns emit? Assume T = 20 $^{\circ} $C Hz