# Tagged Questions

The inclusion-exclusion principle states that the number of elements in the union of two given sets is the sum of the number of elements in each set, minus the number of elements that are in both sets.

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### Famous uses of the inclusion-exclusion principle?

The standard textbook example of using the inclusion-exclusion principle is for solving the problem of derangement counting; using inclusion-exclusion (and some basic analysis) it can be shown that ...
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### Using Inclusion-Exclusion on $n$ boys and $n$ girls

Consider a set of n boys and n girls. In each situation below, use inclusion-exclusion to derive formulas for the number of ways to partition the $2n$ people into pairs so that for each $i$ the $i$-th ...
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### Combinatorics: Using Inclusion-Exclusion Principle

I'm not sure how to start this. I've been told to use inclusion-exclusion, but I don't know which properties to use. Intuitively, I want to solve the problem like this: $E_1$ = 1 person leaves ...
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### When to use Inclusion Exclusion principle?

I'm a bit confused as to when you should use the Incl/Excl principle. Take for example the following problem: How many 13-card hands can be selected from the standard 52-card deck with exactly four ...