SOLUTION: the time in months that a refrigerator remains operational is a normally distributed r.v. with parameters µ = 108 and σ = 20. The manufacturer will replace them the first time

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Question 693423: the time in months that a refrigerator remains operational is a normally distributed r.v. with parameters µ = 108 and σ = 20. The manufacturer will replace them the first time they break. How many months should the manufacturer offer if the they are only willing to replace 0.40% of the refrigerators.
I am not entirely sure where to start with this at all please help.
Answer by stanbon(75887) About Me  (Show Source):
You can put this solution on YOUR website!
the time in months that a refrigerator remains operational is a normally distributed r.v. with parameters µ = 108 and std = 20. The manufacturer will replace them the first time they break. How many months should the manufacturer offer if the they are only willing to replace 0.40% of the refrigerators.
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Draw a normal curve.
Put u = 100
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Find the z-value with a left tail of 0.40% = 0.0040
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invNorm(0.0040) = -2.6521
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Find the corresponding x-value using x = z*s + u
x = -2.6521*20 + 100
x = 46.96 months
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Cheers,
Stan H.
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