A store sells two models of a certain brand of computer. Because of the demand, it is necessary to stock at least twice as many units of model $A$ as units of model $B.$ The cost to the store for the two models is $$800$ and $$1200,$ respectively. The management does not want more than $$20,000$ in computer inventory at any one time, and it wants at least four model A computers and two model B computers in inventory at all times. Devise a system of inequalities describing all possible inventory levels, and sketch the graph of the system.