Question

A boy walks to his school at a distance of $6\text{km}$ with constant speed of $2.5{\text{km h}}^{-1}$ and walks back with a constant speed of $4{\text{km h}}^{-1}$. His average speed for round trip in ${\text{km h}}^{-1}$, is

• A. $\frac{1}{2}$
• B. $3$
• C. $\frac{24}{13}$
• D. $\frac{40}{13}$

Answer 1

The correct option is D $\frac{40}{13}$

Total distance travelled by the boy = 6 km + 6 km = 12 km
Total time taken by the boy $=\frac{6}{2.5}+\frac{6}{4}=2.4+1.5=3.9\text{h}$
$\Rightarrow \text{Average speed}=\frac{12}{3.9}{\text{km h}}^{-1}$
$\therefore \text{Average speed}=\frac{40}{13}{\text{km h}}^{-1}$