Multiple choice

A boat covers 48 km. upstream and 72 km. downstream in 12 hours, while it covers 72 km. upstream and 48 km. downstream in 13 hours. The speed of stream is - एक बोट 12 घंटे में 48 किमी. धारा के विरूद्ध तथा 72 किमी. धारा के सापेक्ष जाती है जबकि 13 घंटे में 72 किमी. धारा के विरूद्ध तथा 48 किमी. धारा के सापेक्ष जाती है। धारा की चाल क्या है?

  1. 2 km./hr./किमी./घं.

  2. 2.2 km./hr./किमी./घं.

  3. 2.5 km./hr./किमी./घं.

  4. 4 km./hr./किमी./घं.
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A Correct answer
Explanation

Let boat speed = b, stream speed = s. Upstream speed = b-s, downstream = b+s. 48/(b-s) + 72/(b+s) = 12 and 72/(b-s) + 48/(b+s) = 13. Adding: 120/(b-s) + 120/(b+s) = 25. Also subtracting gives 24/(b-s) - 24/(b+s) = 1. Solving gives s = 2 km/hr.

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