A man can row a boat at 8 km/h in still water. If the speed of the water current is 2 km/h and it takes him 2 hours to row to a place and come back, then how far off (in km) is the place?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 7.5
Rowing with the current, the speeds add; rowing against it, the current’s speed is taken away. The distance d is the same both ways, and the two times add up to 2 hours.
Downstream speed \(=8+2=10\) km/h; upstream speed \(=8-2=6\) km/h
Time downstream \(+\) time upstream \(=2\) h: \(\displaystyle \frac{d}{10}+\frac{d}{6}=2\)
\(\displaystyle \frac{3d+5d}{30}=2\), so \(8d=60\) and \(d=7.5\) km