A friend of mine came to me the other day with a question about her new car, she asked, my car gets 30 miles per gallon when I drive in the highway and 25 miles when I drive in the city. How many highway miles do I drive on a 400 mile trip if the car consumes on a 400 mile trip if the car consumes 14 gallons of gasolin during the trip?

There are two ways to assign variables for this problem. One way is to use h and c to stand for highway and city miles. The other way is to let h and c stand for highway and city gallon.

I have to use both ways but I don't even know where to start =/
it says to use system of equations

If you let H and C stand for the number of miles driven on the highway or city respectively, then you know:

1. The total miles driven is 400, so:
C miles + H miles = 400 miles.

2. The number of gallons of gas used when driving on the highway is H miles/30 MPG, and the number of gallons used in driving in the city is C miles/25 MPG. So the toatal amount of gas used is:

H/30 + C/25 = 14

You now have two equations in 2 unknowns, so you can solve for H and C.

In doing these type of problems it is always best to write out the units - for example H miles divided by 30 miles per gallon, so that you can see the quantity H/30 is in gallons. This helps to confirm for you that dividing H by 30 is the right way to do it.