Problem 19 (1986 AJHSME)

At the beginning of a trip, the mileage odometer read $56200$ miles. During the trip, the driver filled his tank again with $12$ gallons of gasoline when the odometer read $56560$. At the end of the trip, the driver filled the tank again with $20$ gallons of gasoline. The odometer read $57060$. To the nearest tenth, what was the car's average miles-per-gallon for the entire trip? (Note: The first fill-up of $6$ gallons was to start the trip full).