Question

The ages of commercial aircraft are normally distributed with a mean of 13.5 years and a standard deviation of 7.3 years. What percentage of individual aircraft have ages greater than 15 ​years?

Answer 1

15 is 1.5 above the mean
... this is 1.5/7.3 standard deviations ... .2055

use a z-score table to determine the fraction of the population this represents

15 yr or less ... 58%

Answer 2

Question was 15 years or more.. so the answer is 41.9% or 42%