for a person who drives his car 15000 miles a year trading in a car that gets 15 miles per gallon for a car that gets 18 miles per gallon (a 20% increase) will save them how many gallons of gas each year?
15000/15 - 15000/18 = 166.67
or, since the old car used 1000 gallons, the new car, with 20% (6/5) better mileage uses 5/6 as much gas, saving 1/6 of 1000 = 166.67 gallons.