b
Wall as an observer
Frequency received by wall
${f}_{1}={f}_{0}\left(\frac{{C}}{{C}-{V}}\right)$
Again wall as a source
Frequency received by observer on car
${f}_{2}={f}_{1}\left(\frac{{C}+{V}}{{C}}\right)$
$500=400\left(\frac{{C}+{v}}{{C}-{V}}\right)$
$\frac{5}{4}=\frac{{C}+{V}}{{C}-{V}}$
${C}=9 {V}$
${V}=\frac{{C}}{9}=\frac{330}{9}\,{m} / {s}$
${V}=\frac{330}{9} \times \frac{18}{5}=132\, {km} / {hr}$
