Answer
$524Hz$
Work Step by Step
The frequency heard by the motorcyclist can be determined as
$f^{\prime}=(\frac{1-u_{\circ}/v}{1-u_s/v})f$
We plug in the known values to obtain:
$f^{\prime}=(\frac{1-13.0/343}{1-27.0/343})(502)$
$f^{\prime}=524Hz$
