A boat covers a distance 24 km upstream and returns to the same point in a total of 4 hours. If the speed of boat in downstream is twice its speed in upstream, then the speed of boat in upstream is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 9 km/h
Let the upstream speed be x km/h, so the downstream speed is 2x km/h. Using time = distance ÷ speed for the two 24 km legs:
\( \displaystyle \frac{24}{x} + \frac{24}{2x} = 4 \)
\( \displaystyle \frac{24}{x} + \frac{12}{x} = 4 \)
\( \displaystyle \frac{36}{x} = 4 \)
\(x = 9\)
So the upstream speed is 9 km/h (and the downstream speed is 18 km/h).