Question

A car travels the first one third of a certain distance with a speed of 10 km/hr, the next one third 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

• A. 18 km/hr
• B. 24 km/hr
• C. 30 km/hr
• D. 36 km/hr

Answer 1

Answer: (A)

Let the total distance travelled be x km Then total time taken = $\frac{x/3}{10}+\frac{x/3}{20}+\frac{x/3}{60}=\frac{x}{30}+\frac{x}{60}+\frac{x}{180}$
$=\frac{6x+3x+x}{180}=\frac{10x}{180}=\frac{x}{18}$ hrs Then average speed = $=\frac{Totaldistancetravelled}{Totaltimetaken}$
$=\frac{x}{x/18}$ km/hr = 18 km/hr