Last minute REA study question

Forum for the GRE subject test in mathematics.
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Last minute REA study question

Postby marco » Fri Oct 12, 2012 6:30 pm

(Exam II Prob 60) A biased coin is tossed repeatedly until the first “tail” occurs. The expected number of tosses required to produce the first tail is estimated as T. Assuming this is true, find the probability of at least two tails in 3T tosses.
(A) T3T −(T −1)3T−1(4T) T3T
(B) T3T −(T −1)3T−1(3T) T3T
(C) T3T −(T −1)3T−1(3T −1) T3T
(D) T3T −(T −1)3T−1(4T −1) T3T
(E) None of these

how do you solve this one?

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