The upstream speed of a boat is 75% of its downstream speed. The stream flows at 3 km/hr. What is the speed of the boat in still water?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 21 km/hr
The downstream speed is more than the upstream speed by twice the stream’s speed, that is, by 6 km/hr. Here that gap is the missing 25% of the downstream speed.
Let the downstream speed be \(D\); upstream speed \(=0.75D\)
\(D-0.75D=2\times3\), so \(0.25D=6\) and \(D=24\) km/hr
Upstream speed \(=0.75\times24=18\) km/hr
Speed in still water \(\displaystyle =\frac{24+18}{2}=21\) km/hr