BRIEF EXERCISE 66 Understanding Cost Behaviour: Mixed Costs [LO1 CC6; LO2 CC7, 8] Which of the following are linear cost functions? Which are mixed cost functions? Given: Y = total costs, X1 = production volume, and X2 = number of batches produced. Y = 4,000 + 3X1 + 45X2 Y = 1,500 + 6X1 + 4X2 Y = 16,780 Y = 4.5X Y = 12,450 + 3.20X2?