Two men, both with rowing speeds of 5 mph in still water, start out rowing on a river at the same time in the morning. Man A rows upstream from point A to point B, while man B rows downstream from point B to point A. They pass each other at noon on the river and continue rowing. Man B, rowing downstream, reaches point A at 4 in the afternoon. Man A, rowing upstream, reaches point B at 9 in the evening.
How fast is the river current?