Total time taken $=\frac{100}{40}+\frac{100}{v}$
Average speed $=\frac{\text { total distance travelled }}{\text { total time taken }}$
$48=\frac{200}{\left(\frac{100}{40}+\frac{100}{v}\right)}$
$48=\frac{2}{\left(\frac{1}{40}+\frac{1}{v}\right)}$
$\frac{1}{40}+\frac{1}{v}=\frac{1}{24}$
$\frac{1}{v}=\frac{1}{24}-\frac{1}{40}=\frac{5-3}{120}=\frac{1}{60}$
$v=60 km / hr$