A man can swim at 3 km/h in still water. The stream’s speed is 2 km/h. How long will he take to swim to a point 10 km upstream and then return to the start (total time in hours)?
Aptitude
Boats and Streams
Difficulty: Easy
Choose an option
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A8 1/3 hrs
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B9 1/5 hrs
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C10 hrs
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D12 hrs
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ENone of these
Answer
Correct Answer: 12 hrs
Explanation
Introduction / Context:Swimming problems mirror boat-and-stream logic. The effective speed differs upstream and downstream due to the current.
Given Data / Assumptions:
- Still-water speed b = 3 km/h
- Current c = 2 km/h
- Upstream speed vu = b − c = 1 km/h
- Downstream speed vd = b + c = 5 km/h
- Upstream distance = 10 km; downstream distance = 10 km
Concept / Approach:Total time = time upstream + time downstream = distance/speed per leg.
Step-by-Step Solution:
Time upstream = 10 / 1 = 10 hTime downstream = 10 / 5 = 2 hTotal time = 12 hVerification / Alternative check:Speeds and distances are straightforward; arithmetic is direct and consistent.
Why Other Options Are Wrong:Any alternative value fails to match both leg times computed from the known effective speeds of 1 km/h and 5 km/h.
Common Pitfalls:Using average speed across unequal leg speeds; the correct method adds individual times, not averages the speeds.
Final Answer:12 hrs.