This paper deals with the assignment of task to the members if the multi-functional staff (each worker is able to perform a given subset of types of tasks) of a work centre, during each period (e.g. one hour) into which the planning horizon (e.g. one shift or one week) can be divided. For each type of task to perform, all workers who can perform the task do so at equal worker efficiencies. There are constraints that, if possible, should be respected. The objective is that the percentage of working time dedicated by each worker to each type of task be as close as possible to reference values. The problem is modelled as a sequence of assignments, in which appropriate values for the cost matrix depend on the results of the previous assignments