An important unit consists of two components placed in parallel. The unit performs saEsfactorily if one of the two components is operaEng. Therefore, only one component is operated at a Eme, but both components are kept operaEonal (capable of being operated) as oAen as possible by repairing them as needed. An operaEng component breaks down in a given period with probability 0.2. When this occurs, the parallel component takes over, if it is operaEonal, at the beginning of the next period.