A motor boat takes 4 minutes to cover 1 km downstream and 6 minutes to cover 1 km upstream. What is the speed of the boat in still water?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 12.5 km/hr
Change each time per km into a speed in km/hr (1 hour \(=60\) minutes). The speed in still water is half the sum of the downstream and upstream speeds.
Downstream speed \(\displaystyle =\frac{60}{4}=15\) km/hr
Upstream speed \(\displaystyle =\frac{60}{6}=10\) km/hr
Speed in still water \(\displaystyle =\frac{15+10}{2}=12.5\) km/hr