‎Nowadays inference based on censored samples has been studied by many researchers‎. ‎One of the most common censoring methods is progressively type II censoring‎. ‎In this model‎, ‎n items are put on the test‎. ‎At each failure times some of the remaining items randomly withdrawn from the test‎. ‎This process continues until for a pre-fixed value as m, ‎failure times of m items are observed‎. ‎For determining the best number for the items on the test different criteria can be considered‎. ‎One of the most important factors that can be considered is the cost criterion‎. ‎In this paper‎, ‎by considering cost function and Weibull distribution for the lifetime of items‎, ‎we find the optimal value for the sample size‎, ‎i.e‎. n‎. ‎In order to evaluate‎, ‎the obtained results one example based on real data is given‎. 

When a system is used, it is often of interest to determine with what probability it will work longer than a pre-fixed time. In other words, determining the reliability of this system is of interest. On the other hand, the reliability of each system depends on the structure and reliability of its components. Therefore, in order to improve the reliability of the system, the reliability of its components should be improved. For this purpose, it is necessary to carry out maintenance operations, which will increase costs. Another way to increase the reliability of systems is to change the location of the components. In this paper, the location of system components and optimal maintenance period are determined by minimizing the costs and maximizing the reliability of a series-parallel system. Finally, a numerical example is presented to evaluate the results in the paper.