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