Sheila has a 12 gallon gas tank in her heavily packed van. On a recent trip she drove a total of 90 miles with the air-conditioner on the entire way. The gas gauge read exactly 3/4 of a tank at the start of the trip, and exactly 1/4 of a tank at the end of the trip. Assume the gas gauge is accurate. How many miles per gallon of gas did she get on this trip?
