4 gallons per hour.
Remember that Time = Distance ÷ Speed.
The car travels 112 miles, which uses 112 miles ÷ 28 mpg = 4 gallons of fuel. This leaves 7 gallons.
The car took 112 miles ÷ 64 mph = 1.75 hours (1 hour 45 mins) to travel the 112 miles, which means the 7 gallons were lost in 1 hour 45 mins.
Therefore the car was losing the fuel at a rate of 7 gallons ÷ 1.75 hours = 4 gallons per hour. QED.