Multiple choice

The speed of the stream is 15 kmph less than the speed of the boat in still water. The boat takes 2 hours 36 minutes to travel 54.6 km in downstream. Find the speed of the boat in still water.

  1. 18

  2. 16

  3. 21

  4. 24
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A Correct answer
Explanation

Let boat speed = b, stream speed = s. Given s = b - 15. Downstream speed = b + s = b + (b - 15) = 2b - 15. Distance = 54.6 km, time = 2 hours 36 minutes = 2.6 hours. Speed = 54.6/2.6 = 21 kmph. So 2b - 15 = 21, 2b = 36, b = 18 kmph. Verify: s = 3, downstream speed = 18 + 3 = 21 kmph, time = 54.6/21 = 2.6 hours. Correct.

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