A train travels first one third of the distance on a straight track with velocity$10km h^{-1}$ next one third with velocity $20 kmh^{-1}$ and the last portion with velocity $60 kmh^{-1}$. What is the average velocity of the train for the whole journey ?

Question

$\begin{array}{1 1}(A)\;78\;km\;h^{-1}\\(B)\;18km\;h^{-1} \\(C)\;67\;km\;h^{-1} \\(D)\;45\;km\;h^{-1}\end{array} $