Question

A boy can swim with a speed of \(18\) kmph in still water. If the speed of the water in a river is \(10.8\) kmph, then the average speed of the boy in travelling downstream for a distance of \(120\) m and upstream for a distance of \(90\) m over the river is

• A. \(3\ \mathrm{ms^{-1}}\)
• B. \(3.5\ \mathrm{ms^{-1}}\)
• C. \(4\ \mathrm{ms^{-1}}\)
• D. \(4.5\ \mathrm{ms^{-1}}\)

Hint:

Average speed is always \[ \boxed{\text{Average speed}=\frac{\text{Total distance}}{\text{Total time}}.} \] Compute the time for each part of the journey separately before finding the average speed.

Answer 1

The Correct Option is B