A river flows at 10 miles per hour. A boat traveled with the current from Town A to Town B, which are 390 miles apart. Then, the boat turned around, and traveled against the current to reach Town C, which is 140 miles away from Town B. The second leg of the trip (Town B to Town C) took the same time as the first leg (Town A to Town B). During this whole trip, the boat was driving at a constant still-water speed. Answer the following question: During this trip, the boat's speed on still water was miles per hour.