d
\(f _{1}=\) frequency heard by wall \(= f _{ s }=\left(\frac{ v _{ s }}{ v _{ s }- v _{ c }}\right)\)

\(f _{2}=\) frequency heard by driver after reflection from wall

\(f_{2}=\left(\frac{v_{s}+v_{c}}{v_{s}}\right) f_{1}=\left(\frac{v_{s}+v_{c}}{v_{s}-v_{c}}\right) f_{0}\)

\(\frac{f_{2}}{f_{0}}=\frac{v_{s}-v_{c}}{v_{s}+v_{c}}\)

\(\frac{48}{44}=\frac{v_{s}-v_{c}}{v_{s}+v_{c}}\)

\(12\left( v _{ s }+ v _{ c }\right)=11\left( v _{ s }- v _{ c }\right)\)

\(23 v _{ c }= v _{ s }\)

\(v _{ c }=\frac{ v _{ s }}{23}=\frac{345}{23}=15 m / s\)

\(=\frac{15 \times 18}{5}=54 km / hr\)