Multiple choice

Abhishek can swim at a speed of 12 km per hour in stagnant water. In order to cover the same distance in the river, it took twice as long in upstream than downstream. Calculate the speed of the river.

  1. 4

  2. 7.5

  3. 9.2

  4. 10.5

  5. 7.1
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A Correct answer
Explanation

Let boat speed = 12 km/h and river speed = r km/h. Downstream speed = 12 + r, upstream speed = 12 - r. For same distance d: time ratio is 2:1, so upstream time = 2 × downstream time. Therefore d/(12-r) = 2 × d/(12+r), which gives 12+r = 2(12-r). Solving: 12+r = 24-2r, so 3r = 12 and r = 4 km/h.

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