Calc question? Making a formula out of symbols...MPG concerned?

Question

E= the miles per gallon your car gets

P1= the price at the less expensive gas station

P2= the price at the most expensive gas station

M= the number of additional miles to the less expensive gas

G= the number of gallons of gas you would buy at either station

write a formula for M in terms of E,P1,P2, and G when this is no saving; write a formula in terms of E,P1, P2 and G for the maximum number of miles before it is not cost effective to drive to the cheapest gas station.

Answer 1

If GP1 + M(P1/E) < GP2 then it is cheaper to make the drive.

GP1 = the total you will pay for the cheaper gas...then you add the extra miles, times how much gas cost you per mile, which gives you the total extra you are paying to make the drive.  If all this is less than GP2, which is what you would pay for the more expensive gas without driving the extra miles, then it's cheaper to make the extra drive.  If it's not, you're better off just paying for the more expensive.

Answer 2

check the image for my answer

http://img507.imageshack.us/img507/6946/...

I'm really not sure if I'm right

I think It has to have something with derivatives .. but It doesn't come through my mind now ..

I'm really curious to know what is the right answer

Please I would like to know that If you don't mind to tell me when you have it

Thank you in advance