Multiple choice

A man took 14 hours to row a distance of 48 km downstream and return back to the starting point. He found that he can row 4 km with the stream in the same time as 3 km against the stream. What is the speed of flow of water in the stream?

  1. 1 km/hour

  2. 2 km/hour

  3. 2.5 km/hour

  4. 3 km/hour

  5. 3.5 km/hour
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Let boat speed be b and stream speed be s. Downstream speed = b+s, upstream = b-s. 48/(b+s) + 48/(b-s) = 14. Also, 4/(b+s) = 3/(b-s) implies 4(b-s) = 3(b+s), so 4b-4s = 3b+3s, b = 7s. Substituting b=7s into the time equation: 48/8s + 48/6s = 14, 6/s + 8/s = 14, 14/s = 14, s = 1.

AI explanation

Since he rows 4 km downstream in the same time he rows 3 km upstream, the ratio of downstream speed to upstream speed is 4 to 3. Let the downstream speed be 4y and the upstream speed be 3y. Using the total time formula of 48 / (4y) + 48 / (3y) = 14, we solve to find y equals 2. The speed of the stream is half the difference of the downstream and upstream speeds, so (8 - 6) / 2 equals 1 km/hour.

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