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 15 miles per gallon of gas. during one particular week, the two cars went a combined total of 850 miles, for a total gas consumption of 50 gallons. how many gallons were consumed by each of the two cars that week?

Answer 1

Let the number of gallons consumed be the first car be x, and that of the second car, y. Then:
x + y = 50 . . . . . . . (1)
20x + 15y = 850. . . (2)

(1) x 20 => 20x + 20y = 1000 . . . (3)
(2) - (3) => -5y = -150 => y = -150/-5 = 30

From (1), x + 30 = 50 => x = 50 - 30 = 20

Therefore, first car consumed 20 gallons and the second consumed 30 gallons.