Answer:
[tex]f_2 = 18.5 kHz[/tex]
Explanation:
As the two whistle are played simultaneously
we know that we will hear beats in that case
so we can say
[tex]f_{beat} = f_1 - f_2[/tex]
so here we will have
[tex]f_1 = 23.5 kHz[/tex]
also we know that the shrill that we feel when both whistle are sounded
[tex]f_{beat} = 5000 Hz[/tex]
so the unknown frequency is given as
[tex]f_{beat} = 23.5 kHz - f_2[/tex]
[tex]5000 = 23.5 - f_2[/tex]
[tex]f_2 = 18.5 kHz[/tex]