A man can row a boat at 6 km/hr in still water, and the speed of the stream is 2 km/hr. He rows a certain distance downstream and comes back the same distance upstream in 90 minutes in all. What is the distance?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 4 km
The total time is the time downstream plus the time upstream for the same distance, each found with its own speed.
Downstream speed \(=6+2=8\) km/hr; upstream speed \(=6-2=4\) km/hr
Let the distance be \(d\) km; total time \(\displaystyle =\frac{d}{8}+\frac{d}{4}=\frac{3d}{8}\) hr
90 min \(\displaystyle =\frac{3}{2}\) hr, so \(\displaystyle \frac{3d}{8}=\frac{3}{2}\)
\(d=4\) km