Two Cars (algebra)

Question

A family has two cars. The first car has a fuel efficiency of 15 miles per gallon of gas and the second has a fuel efficiency of 20 miles per gallon of gas. During one particular week, the two cars went a combined total of 825 miles, for a total gas consumption of 45 gallons. How many gallons were consumed by each of the two cars that week?

Answer 1

15F + 20S = 825

15F + 20(45-F) = 825

15F -20F = 825 -900

5F = 75

F =75/5 = 15 gallons for the 1st car with 15mpg

S= 45-15= 30 gallons for the 2nd car with 20 mpg

15(15) + 20(30) = 225 + 600 = 825 miles