Question

A car travels from $A$ to $B$ at a speed of $20km/h$ and returns at a speed of $30km/h$. The average speed of the car for the whole journey is:

• A. $5km/h$
• B. $24km/h$
• C. $25km/h$
• D. $50km/h$

Answer 1

Answer: (B)

$24km/h$

Let the distance between A and B be d ,

Time taken to go from A to B is $t=\frac{d}{20}$

Time taken to go from B to A is $t^{\prime }=\frac{d}{30}$

The average speed of the car is given by =Total distance/Total time=

$\frac{2d}{t+t^{\prime }}=\frac{2d}{\frac{d}{20}+\frac{d}{30}}=24km/h$