A boat moves at 9 km/hr in still water, and the stream flows at 3 km/hr. The boat covers a 24 km stretch of the river once downstream and once upstream. How much more time does the upstream trip take than the downstream trip?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 2 hours
The stream helps the boat downstream and slows it upstream, so the two trips over the same 24 km take different times.
Downstream speed \(=9+3=12\) km/hr; time \(\displaystyle =\frac{24}{12}=2\) hours
Upstream speed \(=9-3=6\) km/hr; time \(\displaystyle =\frac{24}{6}=4\) hours
Extra time upstream \(=4-2=2\) hours