The speed of water current is half the speed of a motor boat. The motor boat travels 15 km downstream in 1 hour. Then the time taken to return to the starting point is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3 hours
Let the speed of the boat in still water be b km/hr; the current is \(\frac{b}{2}\) km/hr. The downstream speed is the sum of the two:
\( \displaystyle b + \frac{b}{2} = 15 \)
\( \displaystyle \frac{3b}{2} = 15 \)
\(b = 10\)
So the current is 5 km/hr. To return, the boat goes upstream, at the difference of the two speeds:
\(\text{Upstream speed} = 10 - 5 = 5 \text{ km/hr}\)
\( \displaystyle \text{Time} = \frac{15}{5} = 3 \text{ hours} \)