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Unformatted text preview: Basic Probability Summer 2011 NYU Courant Institute Midterm Exam with Solutions 1. Suppose that an airplane engine will fail, when in flight, with probability 1 p indepen dently from engine to engine; suppose that the airplane will make a successful flight if at least 50% of its engines remain operational. If p = 3 / 4, which is preferable, a fourengine plane or a twoengine plane? What about if p = 1 / 2? Let X be the binomial variable which is the number of engines that dont fail. For the first part we have p = 3 / 4 and n = 4 and we want P ( X 2) = P ( X = 2) + P ( X = 3) + P ( X = 4) = 4 2 (3 / 4) 2 (1 / 4) 2 + 4 3 (3 / 4) 3 (1 / 4) 1 + 4 4 (3 / 4) 4 = . 9492 . For the two engine plane we have n = 2 and we want P ( X 1) = P ( X = 1) + P ( X = 2) = 2 1 (3 / 4) 1 (1 / 4) 1 + 2 2 (3 / 4) 2 = . 9375 So the fourengine plane is more likely to have a successful flight. Now, what if p = 1 / 2?...
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This note was uploaded on 10/28/2011 for the course MATH 155 taught by Professor Hahe during the Spring '11 term at CUNY Hunter.
 Spring '11
 HaHe
 Probability

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