Solve a minimization problem

Hi, this could be a very simple problem, but I don't know how to do...

Let's say I have 3 sources of income:

Source 1 = $200

Source 2 = $700

Source 3 = $400

Now, I have something that I want to pay for using these income sources, let's say it's $800.

I want to take as small percentage as possible from each source to pay for those $800. So I have something like;

x1*200 + x2*700 + x3*400 = 800