Question

A boy walks to his school at a distance of $9km$ with constant speed of $3km{h}^{-1}$ and walks back with a constant speed of $6km{h}^{-1}$. His average speed for round trip in ($km{h}^{-1}$) is

• A. $3$
• B. $4$
• C. $5$
• D. $6$

Answer 1

The correct option is B $4$

Total distance travelled by the boy $=9km+9km=18km$

using
$\text{time taken}=\frac{\text{distance travelled}}{\text{speed}}$

Total time taken by the boy for to and fro journey $=\frac{9}{3}+\frac{9}{6}=3+1.5=4.5h$
$\text{Average speed}=\frac{\text{Total distance travelled}}{\text{Total time taken}}$
$\Rightarrow \text{Average speed}=\frac{18}{4.5}km{h}^{-1}$
$\therefore \text{Average speed}=4km{h}^{-1}$