An otter is floating down a river. The river has a current speed of 1 mile per hour (mph), and so the otter is also moving at a speed of 1 mph. At some point, the otter passes by a team of rowers rowing against the current, in the opposite direction of the otter. At some point, the rowers turn around and start rowing downstream. Four hours after passing each other the first time, the rowers and the otter pass each other again at the boathouse.
In still water, the rowers can row at a speed of 4 mph. How many miles did the rowers travel upstream between the time they first passed the otter and the time they turned around?
