A motor boat takes 6 hours to go 48 km upstream. After its engine is replaced, its speed in still water becomes double, and it now goes the same 48 km upstream in 2 hours. The river flows at the same speed both times. What is the speed of the river?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 8 km/hr
Let the old speed in still water be \(b\) and the river’s speed be \(r\). Each upstream run gives one equation, and subtracting them removes the river’s speed.
Old upstream speed \(\displaystyle =\frac{48}{6}=8\), so \(b-r=8\)
New upstream speed \(\displaystyle =\frac{48}{2}=24\), so \(2b-r=24\)
Subtracting: \(b=24-8=16\) km/hr
Speed of the river \(r=16-8=8\) km/hr