Multiple choice

In still water, a man can swim at the rate of 9 km/hr. He takes twice the time to swim opposite to the stream in comparison to the direction of the stream. What is the flow rate of the stream?

  1. 2 km/hr

  2. 3 km/hr

  3. 4 km/hr

  4. 5 km/hr

  5. 6 km/hr
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let man speed = M = 9, stream speed = S. Downstream speed = 9+S, upstream = 9-S. Time = Distance / Speed. Time_up = 2 * Time_down -> D/(9-S) = 2 * D/(9+S). 9+S = 18 - 2S -> 3S = 9 -> S = 3 km/hr.

AI explanation

Because the man takes twice the time to swim against the stream compared to with the stream, his upstream speed is exactly half of his downstream speed. Letting the stream's flow rate be y, the equation for the speeds is 9 minus y equals half of 9 plus y. Multiplying both sides by 2 gives 18 minus 2y equals 9 plus y; solving for y results in a flow rate of 3 km/hr.

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