My Math Forum Hyp distribution question

 December 23rd, 2009, 02:28 AM #1 Newbie   Joined: Dec 2009 Posts: 2 Thanks: 0 Hyp distribution question A tutor is checking a practicant's work (declarations) and chooses 3 out of 12 declarations. There are 2 faulty ones. N=12, n=3 S=2 P(X=0) = 12/22 P(X=1) = 9/22 P(X=2) = 1/22 E(X)=0.5 V(X)=15/44 How many declarations must the tutor check, in order for his probability to find at least one faulty declaration to be at least 2/3? Btw, answer in book says n>=5, any help?
 December 23rd, 2009, 02:10 PM #2 Senior Member   Joined: Dec 2009 From: Las Vegas Posts: 209 Thanks: 0 Re: Hyp distribution question Hi; The book is right. For five picks we can get 2 wrong 3 right or 1 wrong 4 right. $\frac{{10 \choose 4} {2 \choose 1}}{{12 \choose 4+1}}+\frac{{10 \choose 3} {2 \choose 2}}{12 \choose 3+2}=\frac{15}{22}$

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