A river flows at 2 km/hr. A motor boat on this river goes 35 km downstream in exactly the time it takes to go 21 km upstream. What is the speed of the motor boat in still water?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 8 km/hr
When the times are equal, the distances are in the same ratio as the speeds.
Let the speed in still water be \(b\) km/hr; downstream speed \(=b+2\), upstream speed \(=b-2\)
\(\displaystyle \frac{b+2}{b-2}=\frac{35}{21}=\frac{5}{3}\)
\(3b+6=5b-10\), so \(2b=16\) and \(b=8\) km/hr
Check: \(35\div10=3.5\) hr and \(21\div6=3.5\) hr