In all combinatorial problems, there are several key concepts that must be distinguished:
The population is the set of elements that is being studied. The number of elements of this set is denoted by the letter n.
The sample is a subset of the population. The number of elements of a sample is denoted by the letter k.
The samples are determined by two aspects:
If the order matters there are permutations.
If the order does not matter there are combinations.
If the elements are repeated there are permutations with repetition or combinations with repetition.
Factorial of a Natural Number
The factorial of a natural number is the product of "n" consecutive factors from “n” to 1. The factorial of a number is denoted by n!.
Calculate the factorial of 5.