SOLUTION: The mean lifetime of a wristwatch is 25 months, with a standard deviation of 5 months. If the distribution is normal, for how many months should a guarantee be made if the manufact

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Question 442736: The mean lifetime of a wristwatch is 25 months, with a standard deviation of 5 months. If the distribution is normal, for how many months should a guarantee be made if the manufacturer does not want to exchange more than 10% of the watches? Assume the variable is normally distributed.
Answer by stanbon(75887) About Me  (Show Source):
You can put this solution on YOUR website!
The mean lifetime of a wristwatch is 25 months, with a standard deviation of 5 months. If the distribution is normal, for how many months should a guarantee be made if the manufacturer does not want to exchange more than 10% of the watches? Assume the variable is normally distributed.
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Find the z-value with a left tail of 10%.
invNorm(0.1) = -1.2816
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Find the corresponding x-value:
x = zs+u
x = -1.2816*5+25
x = 18.59 months
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Guarantee them for 18.59 months. Only
10% will have failed by that time.
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Cheers,
Stan H.