Permutations with At Least One Restriction

Rarely Tested

Permutations with At Least One Restriction

## Formula

If $$a$$ represents the total arrangements and $$b$$ represents arrangements violating the restriction:

$$\text{Required arrangements}=A-B$$

For multiple restrictions, use inclusion-exclusion:

$$N=N_0-\sum N_1+\sum N_2-\sum N_3+\cdots$$

## Usage

- Used when arrangements are subject to one or more restrictions and complementary counting is convenient.

Question 1

Five gentlemen check their hats and coats at a theater. Due to a mix-up, the attendant returns the 5 hats and 5 coats completely at random to the 5 men (one hat and one coat per man).

How many ways can the items be distributed such that no one receives both their own hat and their own coat?

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