-1-Stat 231 Exam IOctober 6, 1997Professor Vardeman1. The random variable the number of hours till failure of a WD disk drive\ œis 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. Useyour answer to (a) and an assumption of independence of the 10 drive lifetimes and evaluate theprobability that at least 9 of these drives are failure-free through 20,000 hours. No need tosimplify. (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 tothe following schedule:Failure in the first 10,000 hours produces a payment of $300Failure at hours (10,00020,000) produces a payment of $3002B B ÐÑB"!ß!!!Failure at hours with 20,000 produces no paymentBB Set up (but do not attempt to evaluate) an expression for the mean warranty payment under thisplan. (This is for an appropriate function .)E2Ð\Ñ2
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