Multiple choice

In a fixed time, a boy swims thrice the distance along the current that he swims against the current. If the speed of the current is 4 km/h, then the speed of the boy in still water is

  1. 8 km/h

  2. 6 km/h

  3. 7 km/h

  4. 9 km/h

  5. None of these
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A Correct answer
Explanation

Let speed in still water be x, current be 4. Downstream = x+4, Upstream = x-4. Distance = (x+4)*t, Distance = (x-4)*3t. (x+4)t = 3(x-4)t. x+4 = 3x-12. 2x = 16, x = 8.

AI explanation

Let the boy's speed in still water be x and the time be t; the downstream speed is (x + 4) and the upstream speed is (x - 4). Because distance equals speed multiplied by time, we have (x + 4)t = 3(x - 4)t. Dividing both sides by t leaves x + 4 = 3x - 12, so 16 = 2x and x = 8 km/h.

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