A man can row a boat at a speed of 8 km/hr. in still water. If the speed of water current is 2 km/hr. and it takes the man 2 hours to row the boat to a certain place and come back, how far is the place from the starting point?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 7.5 km
Going with the current the boat gains the speed of the water, and going against it the boat loses it. The two times add up to 2 hours, which gives one equation in the distance \(d\).
Downstream, with the current: \(8+2=10\) km/h
Upstream, against the current: \(8-2=6\) km/h
Time going plus time coming back: \(\displaystyle\frac{d}{10}+\frac{d}{6}=2\)
\(\displaystyle\frac{3d+5d}{30}=2\), which gives \(8d=60\)
So the place is \(d=7.5\) km away