b
Total distance travelled \(=200 m\)

Total time taken \(=\frac{100}{40}+\frac{100}{v}\)

Average speed \(=\frac{\text { total distance travelled }}{\text { total time taken }}\)

\(48=\frac{200}{\left(\frac{100}{40}+\frac{100}{v}\right)}\)

\(48=\frac{2}{\left(\frac{1}{40}+\frac{1}{v}\right)}\)

\(\frac{1}{40}+\frac{1}{v}=\frac{1}{24}\)

\(\frac{1}{v}=\frac{1}{24}-\frac{1}{40}=\frac{5-3}{120}=\frac{1}{60}\)

\(v=60 km / hr\)