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. thee average speed of the car for the whole journey is :

Answer: Option A

Let the distance travelled by a car be x km
First $$\frac{x}{3}$$ km distance cover at speed of 10 km/hr
Second $$\frac{x}{3}$$ km distance cover at speed = 20 km/hr
Third $$\frac{x}{3}$$ km distance cover at speed = 60 km/hr
According to the question,
Total time ,
$$\begin{array}{rl}& =\frac{x}{3\times 10}+\frac{x}{3\times 20}+\frac{x}{3\times 60}\\ & =\frac{x}{30}+\frac{x}{60}+\frac{x}{180}\\ & =\frac{6x+3x+x}{180}\\ & =\frac{10x}{180}\\ & =\frac{x}{18}\end{array}$$
Average speed :
$$\begin{array}{rl}& =\frac{x\times 18}{x}\text{ km/hr}\\ & =18\text{ km/hr}\end{array}$$