Its a different type of question.

In a family there are two cars. In a given week, the first car gets an average of 20 MPG, and the second car gets 15 MPG. The two cars combined drive a total of 1000 miles in that week, for a total gas consumption of 60 gallons. How many gallons were consumed by each of the cars that week.

and this is what I did:

x= First car @ 20MPG

y= Second car @ 15MPG

x+y=60

20x+15y=60

-20x -20x

15y=-20x+60

/15 /15 /15

y=-20/14x+4

y=-4/3x+4

20x+15(4/3x+4)=60

20x+20x+60=60

-60 -60

20x+20x=0

40x=0

/40 /40

x=40

y=20