IQ scores are normally distributed with a mean of 100 and a standard deviation of 15. What is the probability a random sample of 40 people have a mean IQ score more than 80? Solution Mean ( u ) =100 Standard Deviation ( sd )=15 Number ( n ) = 40 Normal Distribution = Z= X- u / (sd/Sqrt(n) ~ N(0,1) P(X > 80) = (80-100)/15/ Sqrt ( 40 ) = -20/2.372= -8.4327 = P ( Z >-8.4327) From Standard Normal Table = 1.