Multiple choice

A man can row 18 km/hr in still water. The time taken by him to go upstream is twice the time taken by him to come downstream. Find the speed of the stream.

  1. 6.5 km/hr

  2. 2 km/hr

  3. 5 km/hr

  4. 6 km/hr

  5. None of these
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let v = 18 km/hr and s be stream speed. Downstream speed = 18+s, upstream = 18-s. Time = Distance/Speed. Time_up = 2 * Time_down => D/(18-s) = 2 * D/(18+s). 18+s = 36-2s => 3s = 18 => s = 6.

AI explanation

Let the speed of the stream be x, making the upstream speed (18 - x) and the downstream speed (18 + x). Because the distance is constant, taking twice the time upstream means the upstream speed is half the downstream speed, so 18 - x equals 0.5 multiplied by 18 + x. Solving this equation yields 36 - 2x equals 18 + x, meaning 18 equals 3x, so the speed of the stream is 6 km/hr.

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