A store sells two models of kayaks. Because of the demand, it is necessary to stock at least twice as many units of model A as units of model B. The costs to the store for the two models are $540 and $720, respectively. The management does not want more than $30,000 in kayak inventory at any one time, and it wants at least six model A kayaks and three model B kayaks in inventory at all times.

(a) Find a system of inequalities describing all possible inventory levels. (Use A to represent the number of model A kayaks and B to represent the number of model B kayaks.)