SOLUTION: A major cell phone service provider has determined that the number of minutes that its customers use their phone per month is normally distributed with a mean equal to 445.5 minute

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Question 663575: A major cell phone service provider has determined that the number of minutes that its customers use their phone per month is normally distributed with a mean equal to 445.5 minutes with a standard deviation equal to 177.8 minutes. The company is thinking of charging a lower rate for customers who use the phone less than a specified amount. If it wishes to give the rate reduction to no more than 12 percent of its customers, what should the cut off be?
Answer by stanbon(75887) About Me  (Show Source):
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A major cell phone service provider has determined that the number of minutes that its customers use their phone per month is normally distributed with a mean equal to 445.5 minutes with a standard deviation equal to 177.8 minutes. The company is thinking of charging a lower rate for customers who use the phone less than a specified amount. If it wishes to give the rate reduction to no more than 12 percent of its customers, what should the cut off be?
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Find the z-value with with a left-tail of 12%.
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invNorm(0.12) = -1.1750
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Find the x-value that corresponds:
x = z*s + u
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x = -1.1750*177.8+445.5 = 236.59
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Cheers,
Stan H.