A boat has speed 4 km/h in still water and the stream speed is 2 km/h. It takes 9 hours to go a certain distance upstream. How long will it take to come back the same distance downstream?
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A2 hrs
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B2.5 hrs
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C3.5 hrs
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D3 hrs
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ENone of these
Answer
Correct Answer: 3 hrs
Explanation
Introduction / Context:Given still-water and current speeds, we compute upstream and downstream speeds and transfer time information from one direction to the other for the same distance.
Given Data / Assumptions:
- b = 4 km/h, c = 2 km/h.
- Upstream speed = b - c = 2 km/h; downstream speed = b + c = 6 km/h.
- Upstream time = 9 h over distance D.
Concept / Approach:Distance D = speed * time. Then use D to find the downstream time with faster effective speed.
Step-by-Step Solution: D = upstream speed * time = 2 * 9 = 18 km Downstream time = D / (b + c) = 18 / 6 = 3 h
Verification / Alternative check:Round trip check: 9 h up + 3 h down = 12 h total; numbers are consistent and units align.
Why Other Options Are Wrong:2 or 2.5 h are too short for 18 km at a maximum of 6 km/h; 3.5 h is too long.
Common Pitfalls:Using 4 km/h or 2 km/h directly for downstream; forgetting upstream is slower and downstream is faster; unit mismatches.
Final Answer:3 hrs