Question

A car travelled the first one-third of the distance $d$ at a speed of $5{\text{kmh}}^{-1}$, the second one-third of the distance at a speed of $10{\text{kmh}}^{-1}$ and the last one-third of the distance at a speed of $15{\text{kmh}}^{-1}$. Determine the average speed of the car(in ${\text{kmh}}^{-1}$).

• A. 7.5
• B. 8.18
• C. 12
• D. 13.5

Answer 1

The correct option is B 8.18

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

$\Rightarrow \text{Average speed}=\frac{d/3+d/3+d/3}{\frac{d/3}{5}+\frac{d/3}{10}+\frac{d/3}{15}}$
$=\frac{d}{\frac{d}{15}+\frac{d}{30}+\frac{d}{45}}=8.18\text{km/h}$