A swimmer swims 1.2 km upstream in 18 minutes and swims back the same distance in 9 minutes. What is the speed of the swimmer in still water?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 6 km/h
Since the speeds are wanted in km/h, the times are first changed to hours: 18 minutes \(= 0.3\) hour and 9 minutes \(= 0.15\) hour.
\(\text{Upstream speed} = 1.2 \div 0.3 = 4 \text{ km/h}\)
\(\text{Downstream speed} = 1.2 \div 0.15 = 8 \text{ km/h}\)
The speed in still water is the average of the downstream and upstream speeds.
\(\displaystyle \text{Speed in still water} = \frac{8 + 4}{2} = 6 \text{ km/h}\)
So the swimmer swims at 6 km/h in still water (and the stream flows at 2 km/h).