The bobber
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
The current carry the bobber to you at the same rate of the speed you lost to the current (flowing against your canoe).
Answer
= 14 / (7 - c + c)
= 14 / 7
= 2 hours