Internet Purchases An Internet bookstore charges $\$ 15$ shipping for orders under $\$ 100$ but provides free shipping for orders of $\$ 100$ or more. The cost $C$ of an order is a function of the total price $x$ of the books purchased, given by
$$
C(x)=\left\{\begin{array}{ll}
x+15 & \text { if } x<100 \\
x & \text { if } x \geq 100
\end{array}\right.
$$
a. Find $C(75), C(90), C(100),$ and $C(105)$
b. What do your answers in part (a) represent?