A large company with n employees has a scheme according to which each employee buys a Christmas gift and the gifts are then distributed at random to the employees. What is the probability that someone gets his or her own gift?
The number of ways in which n gifts could be distributed among n employees such that no one gets his / her own gift is computed using the Derangement formula D(n) given as:
Also total number of ways in which n gifts could be given to n employees is given as the permutation of n items at n places = n!
Therefore the required probability that someone gets his or her own gift is computed as:
= 1 - Probability that no one gets his or her own gift
This is the required probability here.
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