A car travels the first one-third of a certain distance with a speed of 10 km/hr, the next one-third distance with a speed of 20 km/hr, and the last one-third distance with a speed of 60 km/hr. The average speed of the car for the whole journey is :

Let the whole distance travelled be x km and the average speed of the car for the whole journey be y km/hr
Then,
$$\eqalign{ & \Leftrightarrow \frac{{\left( {\frac{x}{3}} \right)}}{{10}} + \frac{{\left( {\frac{x}{3}} \right)}}{{20}} + \frac{{\left( {\frac{x}{3}} \right)}}{{60}} = \frac{x}{y} \cr & \Leftrightarrow \frac{x}{{30}} + \frac{x}{{60}} + \frac{x}{{180}} = \frac{x}{y} \cr & \Leftrightarrow \frac{1}{{18}}y = 1 \cr & \Leftrightarrow y = 18{\text{ km/hr}} \cr} $$