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A person can row a boat at 20 km/h in still water. It take three and a half hours to row from P to Q and back. If the speed of the stream is 10 km/h, then what is the time taken by the person to cover thrice more distance upstream?
1. 10.5 hours
2. 12.5 hours
3. 14.5 hours
4. 16.5 hours

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Correct Answer - Option 1 : 10.5 hours

Given:

Speed of boat is 20 km/h

Speed of the current is 10 km/h

Concept:

Speed = Distance/Time

Calculation:

Let us assume that distance between P and Q is D

⇒ 3.5 = D/(20 - 10) + D/(20 + 10)

⇒ 3.5 = D × (1/10 + 1/30)

⇒ D = 3.5 × (30/4) = 26.25 km

⇒ Thrice more distance = 105 km

⇒ The time taken to cover the distance in upstream = 105/(20 - 10) = 10.5 hours

∴ The required result will be 10.5 hours.

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