Multiple choice

The speed of a motor boat in still water is 15 km./hr. It goes 90 km downstream and comes back in a total time of 13 hours 30 minutes. The speed of the stream is स्थिर जल में एक मोटर नाव की गति 15 किमी / घंटा है । यह 90 किमी अनुप्रवाह जाकर वापस आने में कुल 13 घंटे 30 मिनट का समय लेती है । धारा की गति है

  1. 5.5 km/hr./(किमी./घंटा)

  2. 5 km/hr./(किमी./घंटा)

  3. 4.5 km/hr./(किमी./घंटा)

  4. 4 km/hr./(किमी./घंटा)
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B Correct answer
Explanation

Let stream speed = x km/hr. Downstream: 90/(15+x), Upstream: 90/(15-x). Total: 90/(15+x) + 90/(15-x) = 13.5. Substituting x=5: 90/20 + 90/10 = 4.5 + 9 = 13.5 hours. This matches exactly.

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