Arun's speed of swimming in still water is 5 km/hr. He swims between two points in a river and returns back to the same starting point. He took 20 minutes more to cover the distance upstream than downstream. If the speed of the stream is 2 km/hr, then the distance between the two points is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1.75 km
The stream slows him going up and helps him coming down:
\(\text{upstream speed} = 5 - 2 = 3 \text{ km/hr}\)
\(\text{downstream speed} = 5 + 2 = 7 \text{ km/hr}\)
Let the distance be d km. The difference of the two times is 20 minutes = 1/3 hour:
\( \displaystyle \frac{d}{3} - \frac{d}{7} = \frac{1}{3} \)
\( \displaystyle \frac{4d}{21} = \frac{1}{3} \)
\( \displaystyle d = \frac{21}{12} = 1.75 \)
The distance is 1.75 km.