First 160 km = 64 km / hr
Next 160 m = 80 km/ hr
Average speed of 320 km --- ?
$\begin{align*}\text{Total time taken} & = (\frac{160}{64} + \frac{160}{80})hrs \\ & =\frac{9}{2}hrs \end{align*}$
$\begin{align*}\therefore\text{Average speed} & = [300 \times \frac{2}{9}]km/hr \\&= 71.11km/hr\end{align*}$