Bob is a steady but absentminded paddler. He begins his canoe trip canoeing upstream. After 1.5 miles, his hat falls into the river and begins floating down stream, but Bob doesn't even notice. Twenty minutes later he reaches his destination and turns around, continuing his same steady paddling in the downstream direction. He reaches his starting point at the same time his hat does, grabs his hat, and pulls his canoe out of the water. How fast is the river flowing?