Ralph's Mufflers manufactures three different product lines, Model X, Model Y, and Model Z. Considerable market demand exists for all models. The following per unit data apply: Model X Model Y Model Z Selling price $160 $180 $200 Direct materials 60 60 60 Direct labor ($20 per hour) 30 30 40 Variable support costs ($10 per machine-hour) 10 20 20 Fixed support costs 40 40 40 a. For each model, compute the contribution margin per unit. b. For each model, compute the contribution margin per machine-hour. c. If there is excess capacity, which model is the most profitable to produce? Why? d. If there is a machine breakdown, which model is the most profitable to produce? Why? e. How can Ralph encourage her sales people to promote the more profitable model