A log floating freely in a river drifts 6 km in 2 hours. A boat covers 45 km downstream in the same river in 3 hours. How long will the boat take to cover 36 km upstream?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 4 hours
A freely floating log has no speed of its own; it moves only with the current, so its speed is the speed of the stream.
Speed of the stream \(\displaystyle =\frac{6}{2}=3\) km/hr
Downstream speed of the boat \(\displaystyle =\frac{45}{3}=15\) km/hr
Speed of the boat in still water \(=15-3=12\) km/hr
Upstream speed \(=12-3=9\) km/hr
Time for 36 km upstream \(\displaystyle =\frac{36}{9}=4\) hours