AMC 8 1986 Problem 19

At the beginning of a trip, the mileage odometer read $56,200$ miles. The driver filled the gas tank with $6$ gallons of gasoline. During the trip, the driver filled his tank again with $12$ gallons of gasoline when the odometer read $56,560$. At the end of the trip, the driver filled his tank again with $20$ gallons of gasoline. The odometer read $57,060$. To the nearest tenth, what was the car's average miles-per-gallon for the entire trip? $\text{(A)}\ 22.5 \text{(B)}\ 22.6 \text{(C)}\ 24.0 \text{(D)}\ 26.9 \text{(E)}\ 27.5$