Consider the subsets of a set with n elements that have a cardinality of n and n−1. Suppose one of these subsets is chosen at random. What is the expected value of the cardinality of this subset?
The set has n elements.
Total number of subset of that set, which has cardinality n is 1. Since n element can be chosen in 1 ways.
Total number of subset of that set, which has cardinality n-1 is . Since n - 1 element can be chosen in ways.
hence total number of subset is with cardinality n and n-1 is n+1.
hence the probability of choosing set of cardinality n is
and probability of choosing set of cardinality n - 1 is
Suppose x is the cardinalty of the set.
So Expectation is
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