Answer:
70.07 Hz
Explanation:
Since the sound is moving away from the observer then
and
when moving towards observer
With
of 76 then taking speed in air as 343 m/s we have


Similarly, with
of 65 we have

Now

v_s=27.76 m/s
Substituting the above into any of the first two equations then we obtain

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To find average speed, we divide the distance of travel (in this case, 400 metres) by the time she took, 32 seconds. Therefore: 12.5 seconds is her average speed.