Country Workshop manufactures both finished and unfinished furniture for the home. The estimated quantities demanded each week of its rolltop desks in the finished and unfinished versions are $x$ and
$y$ units when the corresponding unit prices are
$$
\begin{array}{l}
p=200-\frac{1}{5} x-\frac{1}{10} y \\
q=160-\frac{1}{10} x-\frac{1}{4} y
\end{array}
$$
dollars, respectively.
a. What is the weekly total revenue function $R(x, y) ?$
b. Find the domain of the function $R$.