Multiple choice

A boat rows downstream 68 km and upstream 45 km in 9 hours. The same boat rows 51 km downstream and 72 km upstream takes 2 hours more. Find the rate of the stream. एक नाव अनुप्रवाह 68 किमी. और 45 किमी. ऊर्ध्वप्रवाह 9 घंटे में जाती है | वहीँ नाव 51 किमी. अनुप्रवाह और 72 किमी. ऊर्ध्वप्रवाह जाने में 2 घंटे अधिक लेती है | धारा की चाल गीता कीजिये |

  1. 4 kmph/किमी./घंटा

  2. 5 kmph/किमी./घंटा

  3. 6 kmph/किमी./घंटा

  4. 8 kmph/किमी./घंटा
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A Correct answer
Explanation

Let boat speed in still water = b, stream speed = x. Downstream speed = b+x, upstream speed = b-x. From first condition: 68/(b+x) + 45/(b-x) = 9. From second: 51/(b+x) + 72/(b-x) = 11. Solving gives b+x = 17, b-x = 9. So x = (17-9)/2 = 4 kmph.

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