Define the decision variables and determine the objective function for the following problem.
A plant has four operators to be assigned to four machines. The time (minutes) required by each worker to produce a product on each machine is shown in the following table:
Machine (min.)
Operator A B C D
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1 10 12 9 11
2 5 10 7 8
3 12 14 13 11
4 8 15 11 9
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The objective is to minimize total time.