Explanation:
so, she drove the 12,000 miles with an SUV that gave her 15 miles per gallon.
that means she used
12000 / 15 = 800 gallons of gasoline.
if she would have used a car that gave her 48 miles per gallon, she wound have needed
12000 / 48 = 250 gallons of gasoline.
so, she would have saved
800 - 250 = 550 gallons of gasoline.