A chemist has three acid solutions at various concentrations. The first is 10% acid, the second is 20% acid, and the third is 40% acid. How many milliliters of each should he use to make 100 mL of 18% solution, if he has to use four times as much of the 10% solution as the 40% solution?

I have no idea how to go about this. I'm sure I could solve the problem if I could just set it up! So can anyone help me set up a system of equations for this problem?

EDIT: I've been contemplating this system:

x + y + z = 100

.1x + .2y + .4z = .18

x = 4z

But I don't know how I'd go about setting up a matrix for that...