A man swimming in a river flowing at 3 km/hr finds that in a given time he can swim thrice as far downstream as he can swim upstream. What will be his speed in still water?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 6 km/hr
Downstream the river helps the swimmer, so his speed is his own speed + the river’s speed; upstream it is his own speed − the river’s speed. In the same time, distance is in proportion to speed, so the downstream speed is three times the upstream speed.
Let his speed in still water be \(s\) km/hr.
Downstream speed \(=s+3\), upstream speed \(=s-3\)
\(s+3=3(s-3)\)
\(s+3=3s-9\)
\(2s=12\), so \(s=\) 6 km/hr