Question

A family has two cars. the first car has a fuel efficiency of 20 miles per gallon of gas and the second has a fuel efficiency of 25 miles per gallon of gas. during one particular week, the two cars went a combined total of 775 miles, for a total gas consumption of 35 gallons. how many gallons were consumed by each of the two cars that week?

Answer 1

Let's call the fuel consumption of the first car "x" and the second car "y"
x + y = 35 is the total fuel consumed.
20x + 25y = 775 is the distance traveled.
Solve the two equations. Skipping a few steps, I get 
5y = 75. Therefore y = 15. So y used 15 gallons of gas. Therefore x used 20 gallons.

20 gallons, 15 gallons.