of course it's ok. it would be a forum if it wasn't

check your answers to see if they make sense.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

car x gets 20 MPG, so if it uses 40 gallons, it means it drove 20*40 = 800 miles

car y gets 15 MPG, so if it uses 20 gallons, it means it drove 15*20 = 300 miles

300 + 800 = 1100 miles. which is not what we are told they drove. so your solution can't be right.

let's start over...

Let be the number of gallons used by the 20 MPG car

Let be the number of gallons used by the 15 MPG car

clearly we have .................(1)

(ah, you got that equation! nice job!)

also, for each gallon, car-x drives 20 miles, so the number of miles car-x drives is 20x, similarly, the number of miles car-y drives is 15y, since the cars drove a total of 1000 miles, we have

....................(2)

(aha! that's where you messed up)

now continue