One car is moving on a straight road covers one third of the distance with $20\;km/hr$ and rest with $60\;km/hr$. The average speed

Question

$\begin{array}{1 1} (a) \;40\;km/hr \\ (b) \;80\;km/hr \\ (c) \;46 \;\frac{2}{3}\;km/hr \\ (d) \;36\;km/hr\end{array}$