Elimination/Sustitution Word Problem, Part trois?

Question

Yes, I know, they keep coming, but they've been getting harder, so. :)

So, what how do you set this problem up into two different equations?

*Low fat milk is 1% milk fat, and whole milk is 4% milk fat. How many ounces of each could you combine to make a 9-ounce mixture of milk with 2% milk fat?

Answer 1

The law of mixtures says:
SUM(percent fat x volume milk)= percent fat in product x volume milk. So let there be x ounces of 1 percent milk and 9-x ounces of 4 percent milk. Substitute,
0.01 x + 0.04 (9-x) = 0.02 (9) Solve for x.