The Camera Shop sells two popular models of digital single lens reflex $($DSLR$)$ cameras $($Camera A Price: $$200,$ Camera B Price: $$300).$ The sales of these products are not independent; if the price of one increases, the sales of the other will increase. In economics, these two camera models are called substitutable products. The store wishes to establish a pricing policy to maximize revenue from these products. A study of price and sales data shows the following relationships between the quantity sold $($N$)$ and price $($P$)$ of each model:

NA $=195-0\mathrm{.}6$PA $+0\mathrm{.}25$PB

NB $=301+0\mathrm{.}08$PA $-0\mathrm{.}5$PB