statistics
posted by Tanya on .
The ages of commercial aircraft are normally distributed with a mean of 13.0 years and a standard deviation of 7.5 years what percentage of individual aircraft have ages greater than 15 years assume that a random sample of 49 aircraft is selected and the mean age of the sample is computed. What percentage of sample means have ages greater than 15 years

For the individual aircraft,
Z = (scoremean)/SD
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion related to the Z score. Multiply by 100.
For the mean,
Z = (scoremean)/SEm
SEm = SD/√n
Use the same table. 
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