In Vietnam, physical education is mostly still taught in a traditional model. Volleyball is one of the most practiced sports in Physical Education classes. The study of this article was to explore students' difficulties in the practice of volleyball in school Physical Education (PE) classes, according to the tactical principles of the sport, and to compare them with teachers' understanding of the discipline. This qualitative study was developed in two stages: a) observation of 14 hours/class PE volleyball classes in two different schools and field diary records of students' difficulties during the game according to the tactical principles of the sport, and b) application of a Linkert scale with active PE teachers to classify such problems (Thomás A.P, et al 2020). In the first stage, the following situations were identified as difficulties, in decreasing order of occurrence in the observations: building the attack, occupying spaces on the court, looking for empty spaces, intending to contact the ball, and directing serves. In the teachers' view, however, difficulties in serves were indicated as students' main problem and, on the other hand, occupying court spaces as the action they performed more easily. On this basis, it is concluded that there are differences between the researcher's and the teachers' views, possibly related to teachers' initial coaching and previous experiences based on the traditional model, whose focus is not on the teaching of tactical principles.

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The issue of preserving fresh food during the transportation is one of the long-standing issues for food distributors. On the other hand, the issue of vehicle routing is one of the most important issues in distribution management. Affecting the quality of food is the distribution process, which reduces the distribution time through vehicle routing plays an important role in reducing food spoilage. Therefore, in this study, the problem of vehicle routing in the distribution of perishable food materials is investigated. The time is different so that the distance is minimized and the capacities and time windows are not violated. This research aims to find facilities, identify the size of each cargo, and establish the most efficient vehicle routes to reduce overall costs at three levels in the event of a system failure. Undoubtedly, the traveling salesman issue is one of the most famous issues in the field of artificial intelligence.

Over the course of the last decades, closed-loop supply chains (CLSC) and reverse logistics issues have attracted increasing attention owing to strict environmental laws, social responsibilities, economic interests, and customer awareness. Hence, the issue of closed-loop supply chain and reverse logistics has emerged as a field of research in the new era. This issue has received much attention because it allows recyclable products to return to their original cycle. Therefore, this study primarily intends to present a mathematical model for designing a supply chain network for recycled products. The multi-stage and multi-period objective function of the closed-loop supply chain is presented to meet that aim. In this chain, dismantling, recycling, and disposal centers are considered. The objective function is to reduce the total cost of the closed-loop supply chain. The results of optimizing the mathematical model demonstrate that this model has the necessary efficiency for use in recycled products.