Mixed-model assembly lines present two issues due to differences in processing times from product types; these issues are the work overload or unfinished work and the useless time or unproductive time. Within this context, we present, in this paper, a new mathematical model for the mixed-model sequencing problem. This model minimizes the costs by lost production and idle productive time. The model also allows processors carry out their workload with a factor activity greater than the normal, in order to reduce the work overload if it is necessary. Obviously it is also considered to provide economic compensation to workers based on their level of activation. Finally, the model is evaluated by a computational experience linked to a real case from the automotive industry.