A family has two cars. The first car has a fuel efficiency of 25 miles per gallon of gas and the second has a fuel efficiency of 20 miles per gallon. During one week, the two cars went a combined total of 1700 miles for a total gas consumption of 75 gallons. How many gallons were consumed by each of the two cars?

Solution:

           First car consumes x gallon and second car consumes y gallon.

           25x + 20y = 1700

              x + y = 75

                   y = 75 - x

           25x + 20(75 - x) = 1700

            5x = 200

              x = 40 gallons

              y = 35 gallons