Solve 3 variables with 2 equations

Here is the problem:

Total sales are $5125 and units sold are 1653. Units can be sold either on sale or not on sale. The price on sale or feature sale price (FSP) was $2.25. How many units were sold on sale?

I can derive 2 formulas, but I have 3 variables to solve. Here's how far I have gotten:

Regular price = RSP

Units sold on Sale = Promoted volume (PV)

Units sold at regular = non-promoted volume (NPV)

Total volume = TV

Equation 1: TV = PV + NPV

Equation 2: Total Sales = (RSP x NPV) + (FSP x PV) or $5125 = (RSP x NPV) + 2.25PV

Given these two equations (or any other equations that you can think of), how can I solve for PV?

Any help will be greatly appreciated.