Fuel Efficiency

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
35
miles per gallon of gas. During one particular week, the two cars went a combined total of
925
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 x = number of miles driven by the first car

      y = number of miles driven by the second car

 

Then:      x +      y = 35

            15x + 35y = 925

 

Multiply the first equation by -15:

 

-15x - 15y = -525

  15x +35y = 925

 

Add the equations:  20y = 400

                                       y = 20    and x = 15

 

ANSWER:  the first car used 20 gallons and the second car used 15                      gallons.