Johnny and his father go fishing at 5:00AM. After piloting their boat 6 km upstream, Johnny writes a letter to his friend, puts the letter in a bottle, and drops it into the river. The bottle flows in the current, and at 7:40AM the bottle reaches the point where Johnny and his father began his trip. If the speed of the boat in still water is 9km/he what is the speed of the current?

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