A boy walks to his school at a distance of 6 km with constant speed of 2.5 km/hr and walks both with a constant speed of 4 km/hr . His average speaks for the round trip is

Question

$\begin{array}{1 1} (a) \;24/13\;km/hr \\ (b) \;40/13\;km/hr \\ (c) \;3\;km/hr \\ (d) \;\frac{1}{2}\;km/hr\end{array} $