Five hundred gallons of 89-octane gasoline is obtained by mixing 87-octane gasoline with 92-octane gasoline.

• Form a system of equations in which one equation represents the total amount of final mixture required and the other represents the amounts of 87- and 92-octane gasoline in the final mixture. Let x and y represent the numbers of gallons of 87- and 92-octane gasoline, respectively.
• Use a graphing utility to graph the two equations in part (a) in the same viewing window. Include the graph. As the amount of 87-octane gasoline increases, how does the amount of 92-octane gasoline change?
• How much of each type of gasoline is required to obtain the 500 gallons of 89-octane gasoline?