Solving a value mixture problem

Question

A family has two cars. The first car has a fuel efficiency of 30 miles per gallon of gas and the second has a fuel efficiency of 15 miles per gallon of gas. During one particular week, the two cars went a combined total of 1425 miles, for a total gas consumption of 65 gallons. How many gallons were consumed by each of the two cars that week?

Answer 1

30x +15y = 1425

30x + 15(65-x) = 1425

30x + 975 -15x = 1425

15x = 1425 - 975 = 450

x = 450/15 = 30 gallons with the car having 30 mph

y=65=30 = 35 gallons with the car having 15 mph

30(30) + 35(15) = 900 + 525 =1425