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Need help with a normal percentile question

Statistics
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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

Standard error = sigma /sqrt (n) = 18/ sqrt (20) = 4.025

z = x-mue/ stud error = 120 - 108 / 4.025

  = 2.98

So the probability that z > 2.98  = 1- z (2.98)

                                                   = 1- 0.9986

                                                   =0.0014

Answer: 0.0014


Apr 12th, 2015

I have answered your question.

Niru

Apr 12th, 2015

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