The car using 20 gallons got 15 miles per gallon.
The car using 25 gallons got 20 miles per gallon.
Let x and y represent the "miles per gallon" of the cars using 20 gallons and 25 gallons, respectively.
... 20x +25y = 800 . . . . . total miles traveled
... x + y = 35 . . . . . . . . . . sum of gas mileage numbers
Subtracting 20 times the second equation from the first gives ...
... (20x +25y) -20(x +y) = (800) -20(35)
... 5y = 100
... y = 20
... x = 35 -y = 15
The cars using 20 and 25 gallons got 15 and 20 miles per gallon, respectively.
