Shyam drives his car 30 km at a speed of 45 km/h and, for the next 1 h 20 m, he drives it at a speed of 51 km/h. Find his average speed (in km/h) for the entire journey.

$$\eqalign{ & {\text{Time}} = 1{\text{ hr }}20{\text{ m}} = \frac{4}{3}{\text{ hr}} \cr & {\text{Speed}} = 51{\text{ km/hr}} \cr & \therefore {\text{Distance}} = \frac{4}{3} \times 51 = 68 \cr & {\text{Average speed}} = \frac{{{\text{Total Distance}}}}{{{\text{Total Time}}}} \cr & = \frac{{30 + 68}}{{\frac{{30}}{{45}} + \frac{4}{3}}} \cr & = \frac{{98}}{{\frac{2}{3} + \frac{4}{3}}} \cr & = 49{\text{ km/hr}} \cr} $$