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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.

Oct 23rd, 2017

The standard deviation of average age of 20  vehicles is 18/sqrt(20) = 4.025 months.

for 120 month the z score is

(120-108)/4.025 = 2.9814

Pr (X>120) = Pr(Z>2,9814) = 1-Pr(Z<2.9814) = 1-0.9986 = 0.0014

Apr 13th, 2015

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Oct 23rd, 2017
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Oct 23rd, 2017
Oct 24th, 2017
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