Multiple choice

A man rows a boat upstream a certain distance and then returns back to the same place. If the time taken by him in going upstream is twice the time taken in rowing downstream, find the ratio of the speed of the boat in still water and the speed of the stream.

  1. 2 : 1

  2. 3 : 2

  3. 5 : 3

  4. 3 : 1
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D Correct answer
Explanation

Let boat speed be b and stream speed be s. Upstream speed = b-s, downstream speed = b+s. Time = Distance / Speed. Given t_up = 2 * t_down, so d/(b-s) = 2 * d/(b+s). b+s = 2b - 2s. b = 3s. Ratio b:s = 3:1.

AI explanation

Because the distance is constant and upstream time is twice the downstream time, the downstream speed is twice the upstream speed. Using the boat and stream formulas, we get (b + s) = 2(b - s), which simplifies to 3s = b. This gives a ratio of boat speed to stream speed of 3 : 1. The result is 3 : 1.

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