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231E1F97

# 231E1F97 - Stat 231 Exam I October 6 1997 Professor...

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-1- Stat 231 Exam I October 6, 1997 Professor Vardeman 1 . The random variable the number of hours till failure of a WD disk drive \ œ is described using an exponential distribution with mean 15,000 hours. (a) Evaluate the probability that a given drive lasts at least 20,000 hours. (b) A new computer network has 10 of these drives installed on computers in the network. Use your answer to (a) and an assumption of independence of the 10 drive lifetimes and evaluate the probability that at least 9 of these drives are failure-free through 20,000 hours. No need to simplify. (If you could not do (a) use the incorrect value of .17.) (c) A WD warranty program pays customers for early failure of drives of this type according to the following schedule: Failure in the first 10,000 hours produces a payment of \$300 Failure at hours (10,000 20,000) produces a payment of \$300 2 B  B  Ð Ñ B "!ß!!! Failure at hours with 20,000 produces no payment B B  Set up (but do not attempt to evaluate) an expression for the mean warranty payment under this plan. (This is for an appropriate function .) E 2Ð\Ñ 2

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