Frequency of the sound heard by the passanger in the car

[desmond iking]

Homework Statement

The police car with its siren emits frequency of 828hz approaching a buliding at 20m/s. . What's the frequency reflected from the buliding ?
the ans is ((340)/(340-20))x 828 = 880hz.
What's the frequncy of the reflected sound incident on the approaching police car?
The ans is ((340+20)/340) x 880 = 931hz.
part iii) find the frequency of beats detected by the driver of the police car?
the ans is 931-880= 51hz

for part iii) why not the frequncy of beats =frequency of the reflected sound incident on the approaching police car(931hz) - frequency of the initial sound source (828hz) = 103hz ?

Homework Equations

The Attempt at a Solution

 

[haruspex]

I believe your answer is correct and the book answer wrong.

 

[mfb]

I agree with haruspex. The book's answer looks odd.