Linear Equations word problem help

1. Oct 28, 2004

Gwen

A certain manufacturing plant burns fossil fuel in its manufacturing process. The fuel that is obtains can be divided into 3 types - low emmission fuel, moderate emmission fuel and high emmission fuel. Federal law requires that high emmission fuel be discarded and that the company can burn at most 20% moderate emmission fuel. Suppose that the company gets its fuel from two different sources. Fuel from source 1 costs $6 per ton and consists on the average of 52% low emmission fuel, 28% moderate emmission fuel and the rest high emmission fuel. Fuel from source 2 costs$7 per ton and consists on the average of 60% low emmission fuel, 10% moderate emmission fuel , and the rest high emmission fuel. The company needs at least 290 tons of usable fuel each day. How much fuel should it buy from each to minumize cost? What is this minimum cost?

HERE IS WHAT I HAVE SO FAR!!!
.52x + .6y is less than or equal to 6(.52) + 7(.6)
.28x + .1y is less than or equal to 6(.28)+ 7(.1)
.2x + .3y is less than or equal to 6(.2) + 7(.3)
x >0 and y>0
6x + 7y = Cost

Here is where i am stuck I am missing something in here somewhere.
Can you lead me in the right direction.

thanks
gwen