Multiple choice

A boat can travel 15 km upstream in 3 hours. If the velocity of the boat becomes half, then the return journey also takes the same time. What is the speed of water flow?

  1. 5 3 kmph

  2. 10 3 kmph

  3. 7 3 kmph

  4. 8 3 kmph

  5. 5 2 kmph
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A Correct answer
Explanation

The upstream speed is 15/3 = 5 km/h. If the boat's own speed is halved, the new downstream speed equals 5 km/h, so solving v - w = 5 and v/2 + w = 5 gives w = 5/3 km/h.

AI explanation

The original upstream speed of the boat is 15 km in 3 hours, which equals 5 kmph. Let the speed of the boat in still water be u and the speed of the stream be v; the upstream speed is u - v = 5. On the return journey downstream, the boat's speed becomes u/2, so the downstream speed is (u/2) + v, and we are told this also equals 5 kmph since it takes 3 hours to travel 15 km. Solving the equations u - v = 5 and (u/2) + v = 5 gives u = 20/3 and v = 5/3. The speed of the water flow is 5/3 kmph.

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