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Need help with a normal percentile question

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The Dmv reports that the average age of a vehicle in Santa Clara County is 9 years old (108 months) with a standard deviation of 18 months. For a random sample of 20 vehicles in Santa Clara County what is the probability that the average age of the vehicles in the sample is more than 10 years old (120 months). 

Give your answer to a decimal rounded to the fourth decimal place.

Apr 12th, 2015

P (x > 120) =                                                                    transform to z-score:

P(z > (120 - 108)/(18/sqrt(20)) ) = 

P(z > 2.9814) = 0.0014                                                   use the table/ calculator for standardized normal distribution

Apr 12th, 2015

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Apr 12th, 2015
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Apr 12th, 2015
Dec 10th, 2016
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