6.1.1. Selection and Arrangement Problems The tasks of combinatorics can be derived from selection or arrangement problems. In many practical and mathematical problems, knowing the number of different combinations of selected elements from a finite set is important. These elements can be numbers, letters, people, objects, experiments, events, etc. We will generally denote them as a1, a2, ..., an. It should be noted that different elements are represented by different labels, while identical elements are always represented by the same label. Two combinations are fundamentally different if they do not contain the same number of elements or if they do not contain exactly the same elements. For example, the combinations a1, a2, a3 and a1, a3, or a1, a2, a3 and a1, a2 are considered different.