Answer:
527Hz
Explanation:
The beat frequency of any two waves is:
Fbeat =|f1 – f2|
i.e Â
for frequency tied to length increase (wavelength increases as length increases, therefore frequency decrease:: Â Â Fbeat = f1+f2
for frequency tied to length decrease wavelength increases as length increases, therefore frequency also increases : Â Â Fbeat = f1-f2
Note that a wavelength increase means a decrease of frequency because v = fλ
Therefore from the question:
Fbeat  =4 beats/s
F2 -523Hz
Fbeat = f1-f2
F1=Fbeat+F2
=523+4
=527Hz
 Â