Algebra Help....Mixture Problem?

okay. This is a system of equations. Let's say x=the 10% solution, and y=the 15% solution. You know: x +y =100. You also know: .10x + .15y = (.12)(100). Do you see where I'm getting this equation from? From the percentages multiplied by the amount in mL's. Now all you have to do is solve for x, which I am sure you will be able to do by yourself. I hope this helped. :)