Multiple choice

A man rowing at the rate of 5 km/hr in still water takes thrice as much time in going 40 km up the river as in going 40 km down the river. What is the rate of flow of the stream?

  1. 1.5 km/hr

  2. 2.5 km/hr

  3. 3.5 km/hr

  4. 4.5 km/hr

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

Let v be the stream speed. Upstream speed is 5-v, downstream is 5+v. Time taken upstream is 40/(5-v) and downstream is 40/(5+v). Given 40/(5-v) = 3 * 40/(5+v), so 5+v = 3(5-v), which simplifies to 5+v = 15-3v, 4v = 10, v = 2.5 km/hr.

AI explanation

Using the formula time equals distance divided by speed, the time to travel 40 km upstream is 40 divided by 5 minus y, and the time downstream is 40 divided by 5 plus y, where y is the stream's rate. Because the upstream time is three times the downstream time, 40 divided by 5 minus y equals 3 times 40 divided by 5 plus y. Solving this gives 5 plus y equals 15 minus 3 y, which results in a rate of flow of 2.5 km/hr.

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