A company makes three sizes of cardboard boxes: small, medium, and large. It costs $$\$ 2.50$$ to make a small box, $$\$ 4.00$$ for a medium box, and $$\$ 4.50$$ for a large box. Fixed costs are $$\$ 8000$$.
(a) Express the cost of making $x$ small boxes, $y$ medium boxes, and $z$ large boxes as a function of three variables: $C=f(x, y, z)$.
(b) Find $f(3000,5000,4000)$ and interpret it.
(c) What is the domain of $f$ ?