Multiple choice

A boat takes 9 hours to travel a distance upstream and 3 hours to travel the same distance downstream. If its speed in still water is 4 km/hr, what is the velocity of the stream?

  1. 2 km/hr

  2. 3 km/hr

  3. 1.5 km/hr

  4. 2.5 km/hr
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A Correct answer
Explanation

Let distance be d and stream speed be v. Upstream: d/(4-v) = 9, downstream: d/(4+v) = 3. Equating distances: 9(4-v) = 3(4+v), so 36-9v = 12+3v, giving 12v = 24, v = 2 km/hr. Option B (3 km/hr) would make upstream speed negative. Option D (2.5 km/hr) and C (1.5 km/hr) don't satisfy the time ratio of 3:1.

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