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An urn has m black balls. At each stage, a black ball is removed and a new ball that is black with
NOTE: The preceding has possible applications to understanding the AIDS disease. Part of the body’s immune system consists of a certain class of cells known as T-cells. There are 2 types of T-cells, called CD4 and CD8. Now, while the total number of T-cells in AIDS sufferers is (at least in the early stages of the disease) the same as that in healthy individuals, it has recently been discovered that the mix of CD4 and CD8 T-cells is different. Roughly 60 percent of the T-cells of a healthy person are of the CD4 type, whereas the percentage of the T-cells that are of CD4 type appears to decrease continually in AIDS sufferers. A recent model proposes that the HIV virus (the virus that causes AIDS) attacks CD4 cells and that the body’s mechanism for replacing killed T-cells does not differentiate between whether the killed T-cell was CD4 or CD8. Instead, it just produces a new T-cell that is CD4 with probability .6 and CD8 with probability .4. However, although this would seem to be a very efficient way of replacing killed T-cells when each one killed is equally likely to be any of the body’s T-cells (and thus has probability .6 of being CD4), it has dangerous consequences when facing a virus that targets only the CD4 T-cells.
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EBK FIRST COURSE IN PROBABILITY, A
- Total marks 14 4. Let X and Y be random variables on a probability space (N, F, P) that take values in [0, ∞). Assume that the joint density function of X and Y on [0, ∞) × [0, ∞) is given by f(x, y) = 2e-2x-y Find the probability P(0 ≤ X ≤ 1,0 ≤ y ≤ 2). (ii) spectively. [6 Marks] Find the the probability density function of X and Y, re- [5 Marks] 111) Are the X and Y independent? Justify your answer! [3 Marks]arrow_forwardTotal marks 17 4. Let (,,P) be a probability space and let X : → R be a ran- dom variable that has Gamma(2, 1) distribution, i.e., the distribution of the random variable X is the probability measure on ((0, ∞), B((0, ∞))) given by (i) dPx(x) = xex dx. Find the characteristic function of the random variable X. [8 Marks] (ii) Using the result of (i), calculate the first three moments of the random variable X, i.e., E(X") for n = 1, 2, 3. Using Markov's inequality involving E(X³), (iii) probability P(X > 10). [6 Marks] estimate the [3 Marks]arrow_forward1. There are 8 balls in an urn, of which 6 balls are red, 1 ball is blue and 1 ball is white. You draw a ball from the urn at random, note its colour, do not return the ball to the urn, and then draw a second ball, note its colour, do not return the ball to the urn, and finally draw a third ball, note its colour. (i) (Q, F, P). Describe the corresponding discrete probability space [7 Marks] (ii) Consider the following event, A: At least one of the first two balls is red.arrow_forward
- 3. Consider the following discrete probability space. Let = {aaa, bbb, ccc, abc, acb, bac, bca, cab, cba}, i.e., consists of 3-letter 'words' aaa, bbb, ccc, and all six possible 3-letter 'words' that have a single letter a, a single letter b, and a single letter c. The probability measure P is given by 1 P(w) = for each weΩ. 9 Consider the following events: A: the first letter of a 'word' is a, B: the second letter of a 'word' is a, C: the third letter of a 'word' is a. answer! Decide whether the statements bellow are true or false. Justify your (i) The events A, B, C are pairwise independent. (ii) The events A, B, C are independent. Total marks 7 [7 Marks]arrow_forwardLet X and Y have the following joint probability density function: fxy(x,y) =1/(x²²), for >>1, y>1 0, otherwise Let U = 5XY and V = 3 x. In all question parts below, give your answers to three decimal places (where appropriate). (a) The non-zero part of the joint probability density function of U and V is given by fu,v(u,v) = A√³uc for some constants A, B, C. Find the value of A. Answer: 5 Question 5 Answer saved Flag question Find the value of B. Answer: -1 Question 6 Answer saved P Flag question (b) The support of (U,V), namely the values of u and vthat correspond to the non-zero part of fu,v(u,v) given in part (a), is given by:arrow_forwardTotal marks 13. 3. There are three urns. Urn I contains 3 blue balls and 5 white balls; urn II contains 2 blue balls and 6 white balls; urn III contains 4 blue balls and 4 white balls. Rolling a dice, if 1 appears, we draw a ball from urn I; if 4 or 5 or 6 appears, we draw a ball from urn II; if 2, or 3 appears, we draw a ball from urn III. (i) What is the probability to draw a blue ball? [7 Marks] (ii) Assume that a blue ball is drawn. What is the probability that it came from Urn I? [6 Marks] Turn over. MA-252: Page 3 of 4arrow_forward
- 3. Consider the discrete probability space with the sample space = {a, b, c, d, e, f, g, h} and the probability measure P given by P(w) for each wEN. Consider the following events: A = {a, c, e, g}, B = {b, c, d, e}, C = = {a, b, d, g}. Decide whether the statements bellow are true or false. Justify your answer! (i) The events A, B, C are pairwise independent. (ii) The events A, B, C are independent. Total marks 6 [6 Marks]arrow_forward2. space Consider the discrete probability space (N, F, P) with the sample N = {W1 W2 W3 W4 W5, W6, W7, W8, W9, W10, W11, W12}, is the power of 2, and the probability measure P is given by 1 P(wi) for each i = 1, 12. 12 Consider the following events: A = {W1, W3, W5, W7, W9, W11}, C = B = {W1, WA, W7, W8, W9, W12}, = {W3, WA, W5, W6, W9, W12}. Decide whether the statements bellow are true or false. Justify your answer! (i) The events A, B, C are pairwise independent. [5 Marks] Total marks 8 (ii) The events A, B, C are independent. [3 Marks]arrow_forwardshould my answer be 2.632 or -2.632?arrow_forward
- 2. Alice has three unfair coins. The first coin shows heads with proba- bility 0.3 and tails with probability 0.7. The second coin shows heads with probability 0.9 and tails with probability 0.1. The third coin shows heads with probability 0.4 and tails with probability 0.6. First, Alice throws a dice. If the dice yields a 1 or 2, Alice tosses the first coin. If the dice yields a 3, Alice tosses the second coin. If the dice yields a 4 or 5, or 6, Alice tosses the third coin. (i) What is the probability of the following event: as a result of throwing the dice and then tossing a coin, tails appear. [7 Marks] Assume a coin shows tails. What is the probability that Alice (ii) tossed (a) the first coin; (b) the second coin; Total marks 11 (c) the third coin? [4 Marks]arrow_forwardIn a Poisson process with A = 500 arrivals per day, find the approximate probability that the thousandth arrival comes between 47.5 and 48.5 hours after the start of observations.arrow_forwardSuppose you generate 300 independent observations X1, ..., X300 from a Uniform(0,3) distribution. a) What is the approximate distribution of X? b) What is the approximate probability that X is less than 1.57? c) What is the approximate probability that the sum S300 = X₁++X300 is between 442 and 483?arrow_forward
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