Question

A train travels with a speed of $60km{h}^{-1}$ from station A to station B and then comes back with a speed $80km{h}^{-1}$ from station B to station A. i) The average speed and ii) the average velocity of the train

• A. $70kmh{r}^{-1}$
• B. $\frac{480}{7}kmh{r}^{-1}$
• C. $70m{s}^{-1}$
• D. $\frac{480}{7}m{s}^{-1}$

Answer 1

Answer: (B)

$\frac{480}{7}kmh{r}^{-1}$

Let distance from A to B$=xkm=$ distance from B to A.

Time taken by train from A to B $=\frac{x}{60}hr$

Time taken by train from B to A $=\frac{x}{80}hr$

Total time taken by train $=\frac{x}{60}+\frac{x}{80}=\frac{7x}{240}hr$

$\text{average speed}=\frac{\text{total distance}}{\text{total time}}=\frac{2x}{\frac{7x}{240}}=\frac{480}{7}km{h}^{-1}$