Multiple choice

The speed of a boat in still water is 11 km/hr. It can go 12 km upstream and return downstream to the engine point in 2 hours 45 minute. What is the speed of stream?

  1. 5 km/hr

  2. 4 km/hr

  3. 3 km/hr

  4. 2 km/hr
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A Correct answer
Explanation

Let stream speed = s km/hr. Upstream speed = 11-s, downstream speed = 11+s. Time upstream + time downstream = 12/(11-s) + 12/(11+s) = 2.75 hours. Solving: 12(11+s + 11-s)/((11-s)(11+s)) = 2.75, so 264/(121-s²) = 2.75. This gives s² = 25, so s = 5 km/hr. The answer must be positive since speed can't be negative.

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