ZZ help dopplers effect PHysics?

[Idrees]

PLZZZ help dopplers effect PHysics?
Two students Jan & Lee Hold Buzzers which operate at different frequencies producing a beat frequenzy of 4 Hertz, When Jan runs away from Lee, Lee hears that beat frequency increaes. Who has higher frequency and why?

 

[rl.bhat]

Beat frequency is the difference in frequencies.
If Lee's frequency is f, Jan's frequency will be (f + 4) Hz or (f - 4) Hz. Jan runs away from Lee. Lee will hear the buzzer of Jan with reduced frequency. So the beat frequency will increases only if Jan's frequency is ------?