Multiple choice

A boat takes 4 hours to travel from a place to X to Y downstream and back from Y to X upstream. If the distance from X to Y is 10.5 km and the speed of the current is 9 km/hr, then the speed of the boat in still water, in km/hr: एक नाव एक स्थान X से Yतक धारा के अनुकूल और वापस Y से X तक धारा के प्रतिकूल जाने में 4 घंटे का समय लेती है। यदि X से Y की दूरी 10.5 किमी है और धारा की गति 9 किमी/घंटा है, तो शांत जल में नाव की गति, किमी/घंटा में:

  1. 10.5

  2. 15

  3. 12

  4. 12.5

  5. 11
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let boat speed in still water be b. Downstream speed = b+9, upstream speed = b-9. Distance = 10.5 km each way. Total time = 10.5/(b+9) + 10.5/(b-9) = 4 hours. Solving: 10.5(b-9) + 10.5(b+9) = 4(b²-81). This gives 21b = 4b²-324, or 4b²-21b-324 = 0. Solving the quadratic gives b = 12 km/hr. Option C is correct.

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