The speed of a man in still water is 12 kmph. The time taken by him to row a certain distance upstream is 2.5 times the time taken by him to row the same distance downstream. What is the speed of the stream?

Using the formula for constant distance, the time ratio of 2.5 to 1 means the downstream speed is 2.5 times the upstream speed. This gives the equation (12 + x) equals 2.5 multiplied by (12 minus x), where x is the stream speed. Expanding yields 12 + x equals 30 minus 2.5x. Solving 3.5x equals 18 gives x equals 36 divided by 7, which is 5 and 1/7 kmph.

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