# University level Statistics homework questions (Bernoulli distribution, joint probability etc,)

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P ag e 2 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 2 of 17 MAST20004 Probability Semester 2, 2020 Problem 1. Let P (z) = c(z + 1)(z + 2)(2z + 1), z 2 R be the probability generating function of a random variable X. (i) Find the constant c. Please simply state the result without any partial steps. This is designed for testing AI marking. (ii) Find the pmf of X. (iii) Compute the mean and variance of X by using the function P (z). Page 2 of 17 pages P ag e 3 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 3 of 17 MAST20004 Probability Semester 2, 2020 (iv) Show that X has the same distribution as the sum of three independent Bernoulli random variables. Problem 2. Let X be a continuous uniform random variable over (0,⇡). (i) Let Y = sinX. Use the second order Taylor expansion of (x) = sinx to approximate E[Y ]. Page 3 of 17 pages P ag e 4 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 4 of 17 MAST20004 Probability Semester 2, 2020 (ii) Let X1, · · · , X100 be independent random variables, all having the same distribution as X does. Let S100 = X1 + · · ·+X100. Use Chebyshev’s inequality to estimate the probability P(|S100 � E[S100]| > 20). (iii) Use the central limit theorem to approximate the same probability as in Part (ii). Page 4 of 17 pages P ag e 5 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 5 of 17 MAST20004 Probability Semester 2, 2020 Problem 3. Let {Xn : n > 1} be an independent sequence of Bernoulli random variables with parameter p = 1/2. Let Sn = X1 + · · ·+Xn. (i) By directly computing moment generating functions, show that Sn d = B(n, 1/2). (ii) By using the method of moment generating functions, show that Sn � E[Sn]p V [Sn] d�! N(0, 1) as n ! 1. Page 5 of 17 pages P ag e 6 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 6 of 17 MAST20004 Probability Semester 2, 2020 Page 6 of 17 pages P ag e 7 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 7 of 17 MAST20004 Probability Semester 2, 2020 Problem 4. Each new book donated to a library needs to be processed by the librarian. Suppose that the time it takes to process a book is a random variable with mean 9 minutes and standard deviation 4 minutes. (i) Suppose that the librarian has 50 books to process. Approximate the probability that it will take more than 480 minutes to process all the books. (ii) Approximate the probability that at least 10 books will be processed in the first 80 minutes. Page 7 of 17 pages P ag e 8 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 8 of 17 MAST20004 Probability Semester 2, 2020 (iii) When solving the above two parts, what assumptions have you made implicitly? Problem 5. In this problem, you may need to use the following so-called inclusion-exclusion principle without proof. Let A1, A2, · · · , An be n events. Then P(A1 [ · · · [An) = nX i=1 P(Ai)� X 16i<j6n P(Ai \Aj) + X 16i<j<k6n P(Ai \Aj \Ak)� · · ·+ (�1)n�1P(A1 \ · · · \An). A little girl is painting on a blank paper. Suppose that there is a total number of N available colours. At each time she selects one colour randomly and paints on the paper. It is possible that she picks a colour that she has already used before. Di↵erent selections are assumed to be independent. (1) Suppose that the little girl makes n selections. (1-i) If red and blue are among the available colours, let R (respectively, B) be the event that her painting contains colour red (respectively, blue). What is P(R) and P(R [B)? Page 8 of 17 pages P ag e 9 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 9 of 17 MAST20004 Probability Semester 2, 2020 (1-ii) Suppose that n = N . For 1 6 i 6 N, let Ei be the probability that her painting does not contain colour i. By applying the inclusion-exclusion principle to the event [Ni=1Ei, show that N ! = NX k=0 (�1)k ✓ N k ◆ (N � k)N . Page 9 of 17 pages P ag e 10 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 10 of 17 MAST20004 Probability Semester 2, 2020 (1-iii) Let D be the number of di↵erent colours she obtain among her n selections. By writing N �D as a sum of Bernoulli random variables, compute E[D] and Var[D]. Page 10 of 17 pages P ag e 11 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 11 of 17 MAST20004 Probability Semester 2, 2020 Page 11 of 17 pages P ag e 12 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 12 of 17 MAST20004 Probability Semester 2, 2020 (2) Let S be the number of selections needed until every available colour has been selected by the little girl. For 0 6 i 6 N � 1, let Xi be the random variable that after obtaining i di↵erent colours, the number of extra selections needed until further receiving a new colour. By studying the distributions of these Xi’s as well as their relationship with S, show that E[S] = N ⇥ � 1 + 1 2 + · · ·+ 1 N � . Page 12 of 17 pages P ag e 13 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 13 of 17 MAST20004 Probability Semester 2, 2020 (3) Let T be the number of selections until the little girl picks a colour that she has obtained previously. (3-i) By using the formula E[T ] = 1X k=0 P(T > k), show that E[T ] = N ! NN NX j=0 N j j! Page 13 of 17 pages P ag e 14 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 14 of 17 MAST20004 Probability Semester 2, 2020 (3-ii) Consider E[T ] as a function of N . Show that E[T ] ⇠ r ⇡N 2 as N ! 1. Here the notation aN ⇠ bN means limN!1 aNbN = 1. [Hint: Try to use the central limit theorem in a suitable context.] Page 14 of 17 pages P ag e 15 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 15 of 17 MAST20004 Probability Semester 2, 2020 If you need additional answer space for any of the above 5 questions, please use the following boxes. Clearly indicate the question number before continuing your solutions. Page 15 of 17 pages P ag e 16 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 16 of 17 MAST20004 Probability Semester 2, 2020 Page 16 of 17 pages P ag e 17 of 17 — ad d ex tr a pa ge s af te r pa ge 17 — P ag e 17 of 17 MAST20004 Probability Semester 2, 2020 End of Assignment Page 17 of 17 pages

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