Seru system balancing: Definition, formulation, and exact solution.

•Define and formulate seru balance to describe workloads balance of the workers in a seru.•Define and formulate Intra-SSB to evaluate workloads balance of all workers.•Define and formulate Inter-SSB to evaluate workloads balance among all serus.•Theoretically develop the lower and upper bounds of Intra-SSB and Inter-SSB.•Define and formulate the seru system balancing problem (SSBP) with maximizing Intra-SSB and Inter-SSB.•Develop an improved exact algorithm based on ε-constraint for SSBP.