Arranging the group of elements in a order is known as Permutation.
suppose we have three letters x, y and z. For each one letters can be written in 3 different ways. The first, second or third letter can be written in 3 possible ways, as shown in figure below.

Selecting and arranging a group of elements in an interval of time is known as Combinations.
Suppose we have four numbers 5, 6, 7 and 8. It can be arranged in 1 way when order does not matter.
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## Permutations and Combinations Formula

Below you could see Permutations and Combinations formula

Permutation formula is P(n, r) = $\frac {n!}{(n-r)!}$
Where n is number of elements and r is number of times.

Combinations formula is C(n, r) = $\frac {n!}{(r!(n-r)!)}$

Where n is number of elements and r is number of times.

Factorial is a positive integer and it is denoted with the symbol 'n!'. It is a product of positive integer 1 to n.

0! = 1
n! = $1 \times 2 \times 3 ...\times n$