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Find a repeat-draw distribution and approximate its sample mean

A bag has three identically shaped balls numbered 1,2,3. Draw one at a time at random with replacement. Let XX be the number of draws until any ball is drawn a second time; matching draws need not be consecutive.

(i) Show P(X=4)=2/9P(X=4)=2/9 and find the distribution of XX. marks

(ii) Show E(X)=26/9E(X)=26/9 and find exact Var⁡(X)\operatorname{Var}(X). marks

(iii) Using a suitable approximation, find the probability that the mean of 44 independent observations exceeds 3. marks

[2017/NJC/II/7]

CJC · Review problems · Question 9

A repeat is guaranteed by the fourth draw; matching draws need not be adjacent.

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