Semiautomatic mixers are used in a paint plant. It takes 6 minutes for an operator to load the appropriate pigments and paint base into a mixer. Mixers run automatically and then automatically dispense paint into 50-gallon drums. Mixing and unloading take 30 minutes to complete. Mixers are cleaned automatically between batches; it takes 4 minutes to clean each mixer. Between batches, an operator places empty drums in the magazine to position them for filling; it requires 6 minutes to load the drums into the magazine.

Filled drums are transported automatically by conveyor to a test area before being stored. Mixers are located close enough for travel between mixers to be negligible.

a. What is the maximum number of mixers that can be assigned to an operator without creating idle time for the mixers?

b. If Co  $12/hr and Cm  $25/hr, what assignment of mixers to operators will minimize the cost per batch produced?