A motorboat can row at the speed of 8 km/hr in still water. If the speed of current is 2 km/hr and the boat takes 4 hours to row to a place and return back, how far is the place?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 15 km
Let the place be d km away. With the current the boat moves faster, against it slower:
Downstream speed = 8 + 2 = 10 km/hr
Upstream speed = 8 − 2 = 6 km/hr
The two trips together take 4 hours:
\( \displaystyle \frac{d}{10} + \frac{d}{6} = 4 \)
Multiplying by 30:
\(3d + 5d = 120\)
\(8d = 120\)
\(d = 15\)
So the place is 15 km away.