A small construction firm specializes in building and selling single-family homes. The firm offers two types of homes: Model A requires 4000 labor hours, 2 tons of bricks and 2000 board-feet of lumber. Model B requires 10,000 labor hours, 3 tons of bricks and 2000 board-feet of lumber. The firm is forced to rely on its existing supplies and human resources for the upcoming building season. The firm presently has 400,000 labor hours, 150 tons of bricks and 200,000 board-feet of lumber available. Model A yields a profit of $3000 per unit while Model B yields a profit of $6000 per unit. Assume that the firm is able to sell all units it can build. What mix of Model A and Model B units should the firm construct to maximize the profit?