Paddle your canoe
You can paddle your canoe seven miles per hour through any placid lake. The stream flows at three miles per hour. The moment you start to paddle up stream a fisherman looses one of his bobbers in the water fourteen miles up stream of you.
How many hours does it take for you and the bobber to meet?
The answer is 2.
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1 solution
If the river is our system of coordinates the velocity of stream doesn't matter.