Chemistry Math

So I am trying to make a program to determine the mass of solute in a solution given the mass and volume of the solution and the density of the solvent. I have decided to make a systems of equations in for variables to do it. So this is what I have:
Let a = mass of solvent
Let b = mass of solute
Let c = volume of solvent
Let d = volume of solute
Let x = mass of solution
Let y = volume of solution
Let z = density of solvent

a + b = x
c + d = y
a / c = z
?????????

What would my fourth equation be? I think it has to be a relation between density of the solvent (z) and the density of the solute (b / d).

Please help me