The speed of the current in a river is one-third of the upstream speed of a boat. The boat covers 40 km downstream in 2 hours. What is the speed of the boat in still water?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 16 km/hr
Write the speed of the current as \(c\); the upstream speed is then \(3c\). The downstream speed is the upstream speed plus twice the current (the current is added back once to reach the still-water speed and once more to go downstream).
Downstream speed \(\displaystyle =\frac{40}{2}=20\) km/hr
Upstream speed \(=3c\), so downstream speed \(=3c+2c=5c\)
\(5c=20\), so \(c=4\) km/hr
Upstream speed \(=3\times4=12\) km/hr
Speed of the boat in still water \(=12+4=16\) km/hr