Multiple choice

A man can row at a speed of 15/2 km/hr in still water. If he takes 4 times as long to row a distance upstream as to row the same distance downstream, then the speed of stream (in km/hr) is

  1. 4

  2. 3.75

  3. 5

  4. 4.5
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let v be the speed of the stream. Downstream speed is 7.5 + v, and upstream speed is 7.5 - v. Since time = distance / speed and time_up = 4 * time_down, we have 1 / (7.5 - v) = 4 / (7.5 + v). Solving this gives 7.5 + v = 30 - 4v, so 5v = 22.5, which means v = 4.5.

AI explanation

Because the upstream time is 4 times the downstream time for the same distance, the downstream speed is 4 times the upstream speed. Let his upstream speed be v, making his downstream speed 4v, and using the still water speed formula we have (v + 4v) / 2 equals 15/2. This gives 5v equals 15, so v is 3 km/hr and the downstream speed is 12 km/hr. The speed of the stream is half the difference of the downstream and upstream speeds, giving (12 - 3) / 2 equals 4.5 km/hr.

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