I don't know what to do on this problem. Does someone have the answer? A retailer plans to sell two models of high-quality headphones. The retailer can purchase the wired model for $350 and the wireless model for $400. The wired model yields a profit of $85 and the wireless model yields a profit of $90. The total demand per month for the two models will not exceed 150. The retailer can invest no more than $56,000 for inventory of these items. Find the number of each model that should be stocked each month in order to maximize the retailer's profit.

The retailer should stock of the wired model and of the wireless model to maximize their profit.