Question

A car travels ${\frac{1}{3}}^{rd}$ distance on a straight road with a velocity of 10 km/hr, next ${\frac{1}{3}}^{rd}$ with velocity 20 km/hr and the last ${\frac{1}{3}}^{rd}$ with velocity 60 km/hr. What is the average speed of the car in the whole journey?

• A. 4 km/hr
• B. 6 km/hr
• C. 12 km/hr
• D. 18 km/hr

Answer 1

The correct option is D 18 km/hr

Let 'd' be the total distance travelled
Time taken for 1st one-third distance $=\frac{d}{30}$ hrs
Time taken for 2nd one-third distance $=\frac{d}{60}$ hrs
Time taken for last one-third distance $=\frac{d}{180}$ hrs
Average speed$=\frac{\text{total distance}}{\text{total time taken}}$
$v=\frac{d}{\frac{d}{30}+\frac{d}{60}+\frac{d}{180}}$
$v=18km/hr$