Question

A car moves from A to B with a speed of $30kmph$ and from B to A with a speed of $20kmph$. What is the average speed of the car?

• A. $25kmph$
• B. $24kmph$
• C. $50kmph$
• D. $10kmph$

Answer 1

Answer: (B)

$24kmph$

$Theaveragespeed$ of any moving body is $totaldistance$ traveled by the body divided by $thetotaltime$ taken to cover that distance.

Let the distance between A to B be $x$.

Journey from A to B :

Time taken $t_1=\frac{Distance}{Speed}=\frac{x}{30}$ h

Journey from B to A :

Time taken $t_2=\frac{Distance}{Speed}=\frac{x}{20}$ h

Total distance covered in whole journey $S^{\prime }=2x$

Total time taken $t^{\prime }=t_1+t_2$

Average speed $v_{avg}=\frac{S^{\prime }}{t^{\prime }}$

$\therefore$ $v_{avg}=\frac{2x}{\frac{x}{30}+\frac{x}{20}}=\frac{2\times 20\times 30}{20+30}=24$ kmph