The production function is f (L, M) = 3L K /4, where L is the number of units of labor and K is the number of machines. If the amounts of both factors can be varied and if the cost of labor is $81 per unit and the cost of using machines is $1 per machine, then the total cost of producing 20 units of output is $6500 $800 O $400 $200 $3200