Weston Publishing publishes a deluxe edition and a standard edition of its Spanish-English dictionary. Weston's management estimates that the number of deluxe editions demanded is $x$
copies/day and the number of standard editions demanded is $y$ copies/day when the unit prices are
$$
\begin{array}{l}
p=20-0.005 x-0.001 y \\
q=15-0.001 x-0.003 y
\end{array}
$$
dollars, respectively.
a. Find the daily total revenue function $R(x, y)$
b. Find the domain of the function $R$