A person covers 40% of the distance from A to B at 8 km/h, 40% of the remaining distance at 9 km/h and the rest at 12 km/h. His average speed (in km/h) for the journey is:

$$\eqalign{ & {\text{Let total distance}} = 100 \cr & {\text{Average speed}} = \frac{{{\text{Total distance}}}}{{{\text{Total time}}}} \cr & = \frac{{100}}{{\frac{{40}}{8} + \frac{{24}}{9} + \frac{{36}}{{12}}}} \cr & = \frac{{100}}{{5 + \frac{8}{3} + 3}} \cr & = \frac{{100 \times 3}}{{32}} \cr & = \frac{{75}}{8} \cr & = 9\frac{3}{8} \cr} $$