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A man can swim in still water at 4.5 km/h, but takes twice as long to swim upstream than downstream. The speed of the stream is_________?

Correct answer: D. 1.5

  • A. 3
  • B. 7.5
  • C. 2.25
  • D. 1.5

Explanation

Let the current speed be c km/h. Since the upstream time is twice the downstream time for the same distance, the downstream speed is twice the upstream speed: 4.5 km/h + c = 2(4.5 km/h - c). Solving gives 3c = 4.5 km/h, so c = 1.5 km/h. The likely mistake is choosing 2.25, half the still-water speed, without using the time condition.

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About Time, Speed and Distance

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