SOLUTION: the manager of a small postal substation is trying to quantify the variation in the weekly demand for mailing tubes. She has decided to assume that this demand is normally distribu

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Question 687845: the manager of a small postal substation is trying to quantify the variation in the weekly demand for mailing tubes. She has decided to assume that this demand is normally distributed. She knows that on average 100 tubes are purchased weekly and that 90 percent of the time, weekly demand is below 115. What is the standard deviation of this distribution?
Answer by stanbon(75887) About Me  (Show Source):
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the manager of a small postal substation is trying to quantify the variation in the weekly demand for mailing tubes. She has decided to assume that this demand is normally distributed. She knows that on average 100 tubes are purchased weekly and that 90 percent of the time, weekly demand is below 115. What is the standard deviation of this distribution?
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u = 100
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Find the z-value with a left tail of 90%
invNorm(0.9) = 1.2816
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solve for "s" using x = z*s + u
115 = 1.2816*s + 100
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15 = 1.2816s
s = 11.7
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Cheers,
Stan H.
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