A car covers 15 km, 20 km, 30 km and 12 km at speeds of 20 km/h, 30 km/h, 40 km/h and 30 km/h, respectively. The average speed of the car for the total journey is:

$$\eqalign{ & {\text{Average speed}} = \frac{{{\text{Total distance}}}}{{{\text{Total time}}}} \cr & = \frac{{15 + 20 + 30 + 12}}{{\frac{3}{4} + \frac{2}{3} + \frac{3}{4} + \frac{2}{5}}} \cr & = \frac{{27}}{{\frac{3}{2} + \frac{2}{3} + \frac{2}{5}}} \cr & = \frac{{77 \times 30}}{{45 + 20 + 12}} \cr & = \frac{{77 \times 30}}{{77}} \cr & = 30 \cr & {\text{Average speed}} = 30{\text{ km/h}} \cr} $$