Multiple choice

A motor boat can travel at 10 km/hour in still water. It travelled 91 km downstream in a river and then returned to the same place taking altogether 20 hours. Find the rate of the flow of the river? एक मोटर बोट शांत जल में 10 किमी/घंटे की चाल से यात्रा कर सकती है। इसे एक नदी में धारा की दिशा में 91 किलोमीटर की यात्रा करने और फिर पुनः अपने स्थान पर वापस आने में 20 घंटे लगते हैं। नदी के प्रवाह की दर ज्ञात कीजिये?

  1. 2 km/hr

  2. 3 km/hr

  3. 5 km/hr

  4. 6 km/hr

  5. 8 km/hr
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B Correct answer
Explanation

Let boat speed = 10 km/hr, let current speed = c. Downstream speed = 10 + c, upstream speed = 10 - c. Time downstream = 91/(10 + c), time upstream = 91/(10 - c). Total time = 91/(10 + c) + 91/(10 - c) = 20. Testing c = 3: downstream speed = 13 km/hr (7 hours), upstream speed = 7 km/hr (13 hours), total = 20 hours. This satisfies the condition exactly. Other options don't satisfy the equation.

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