Two jetties, X and Y, are 54 km apart on a river, with Y downstream of X. At the same moment, a launch leaves X for Y and a ferry leaves Y for X. The launch moves at 10 km/hr and the ferry at 8 km/hr in still water, and the river flows at 3 km/hr. After how long do they meet?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3 hours
The launch goes with the river and the ferry against it. The river adds 3 km/hr to one and takes 3 km/hr from the other, so the gap between them closes at the sum of their still-water speeds.
Launch (downstream) \(=10+3=13\) km/hr
Ferry (upstream) \(=8-3=5\) km/hr
The gap closes at \(13+5=18\) km/hr
Time to meet \(\displaystyle =\frac{54}{18}=3\) hours