# 7. (Sec. 5.4) The lifetime of a certain type of battery is normally distributed with mean value 10 hours and standard deviation 1 hour.(a) If a pack of 4 batteries is purchased, what is the probability that the average lifetime of the batteries in the package is at least 9 hours?(b) How many batteries must be purchased such that the probability that their average lifetime is at least 9.5 hours is .99?

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7. (Sec. 5.4) The lifetime of a certain type of battery is normally distributed with mean value 10 hours and standard deviation 1 hour.

(a) If a pack of 4 batteries is purchased, what is the probability that the average lifetime of the batteries in the package is at least 9 hours?

(b) How many batteries must be purchased such that the probability that their average lifetime is at least 9.5 hours is .99?

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Step 1

Part a)

To calculate the probability that the average lifetime of the batteries in the package is at least 9 hours ie P(X>=9) , we proceed to find the mean and the standard deviation of the pack.

Step 2

Part b)

We have to find n  such...

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