[SOLVED] An Algebra II Problem: Involving A System of Equations I Think

Hey-

Can someone explain how to do this Algebra II problem?

The top two selling DVDs of 2003 grossed a combined total of $600.9 million. The top selling DVD grossed 39.9 million more than the DVD ranked 2nd. How much did the top selling DVD gross?

I know the answer is 320.4 million dollars, but I don't know how the person I got it from, got that answer, so if someone could explain it to me, I would greatly appreciate it. I know the person used an equation, or a system of equations, something like that.

-Isabel