The speed of a boat in still water is 12 km/hr. If it can travel 48 km downstream and 24 km upstream in the same time, the speed of the stream is,
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 4 km/hr
Downstream the stream adds to the boat’s speed and upstream it takes away from it. With the stream’s speed \(s\) km/hr, the two speeds are \(12+s\) and \(12-s\), and the times for the two trips are equal.
The times are equal: \(\displaystyle\frac{48}{12+s}=\frac{24}{12-s}\)
This gives \(48(12-s)=24(12+s)\), or \(24-2s=12+s\)
\(3s=12\), so \(s=\) 4 km/hr