A furniture manufacturer produces tables, desks and chairs. The profits per item are, respectively, $55,$70 and $50. The pieces of furniture require the following numbers of labor-hours for their manufacture: The following amounts of labor are available per month: carpentry, at most 2412 hours; painting, at most 6690 hours; and sanding, at most 4380 hours. The manufacturer wants to maximize the profit. Now, suppose the manufacturer is considering also making book cases which require 5 hours of carpentry, 12 hours of painting and 15 hours of sanding. What profit must he gain per book case to justify adding book cases to his product line? (We consider it justifiable to add book cases to the product line when the production schedule that maximizes the profit involves making a positive number of book cases)