Answer
The girl hears a frequency of $~~500.0~Hz$
Work Step by Step
In this situation:
$f = 500.0~Hz$
$v_D = 20.00~m/s$
$v_S = 20.00~m/s$
We can find the frequency that the girl hears:
$f' = f~\frac{v+v_D}{v+v_S}$
$f' = (500.0~Hz)~(\frac{343~m/s+20.00~m/s}{343~m/s+20.00~m/s})$
$f' = 500.0~Hz$
The girl hears a frequency of $~~500.0~Hz$
