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Question 3 (20 marks): Redundancy is a method against hard drive failure. A redundancy array of 4 hard drives works normally as long as 2 or more hard drives are working. Each hard drive has a probability of failure p. Let X is a random variable that records the number of working hard drives. (3.) Find the probability mass mction of random variable px(a:) = P[X 2 k], k 6 3X. (10 marks) (b) Find the probability of failure pm of the redundancy array. (10 marks)