Derangement | Formula Proof and Practice Questions (Permutations and Combinations)

Derangement or De-arrangement is an important concept in Permutations and Combinations.
This video lesson illustrates the concept and covers its proof through recursion using the classic-hat check scenario: Find the number of ways in which n hats can be returned to n people, such that no hat makes it back to its owner.

The following actual questions from previous papers have additionally been discussed to make the application of Derangement crystal clear. Drop in any comments/queries regarding this topic in the comments box below.
Question 1: A professor gave a test to 4 students – A, B, C, and D – and wants to let them grade each other's tests. How many ways could the professor hand the tests back to the students for grading, such that no student received their own test back?
Question 2: If we place 6 letters into 6 addressed envelopes, what’s the probability that exactly 2 of them will be placed correctly?

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[appalaraju]

Thank you for your uploading this video
Way of teaching next level

[RamSingh-itvy]

your explanation is awesome, thank you so much mam continue this type of video