Question

One car moving on a straight road cover one third of the distance with $20km/hr$ and the rest with $60km/hr$. The average speed of the car is

• A. $40km/h$
• B. $80km/h$
• C. $46\frac{2}{3}km/h$
• D. $36km/h$

Answer 1

Answer: (D)

$36km/h$

Let $3dkm$ be total distance.
Time taken for first one third $t_1=\frac{d}{20}hr$

Time taken for rest of journey $t_2=\frac{2d}{60}hr$

Total time $T=\frac{d}{20}+\frac{2d}{60}=\frac{5d}{60}=\frac{d}{12}$

Average speed $=\frac{3d}{\frac{d}{12}}=36km/hr$