Adult IQ percentage

Question

Assume that adults have IQ scores that are normally distributed with a mean of =100 and a standard deviation =20 . Find the probability that a randomly selected adult has an IQ less than 128

Answer 1

mean = 100, standard deviation = 20

Pr(x<128) = 91.9243%= about 92%

find the z score, z=(128-100)/20 = 28/20 = 14/10 = 1.4 standard deviations above the mean

use a z calculator, online or handheld, or use z tables

Pr(x>128) = .080757

Pr(x<128)=1-Pr(x>128) = 1-.080757 = .919243 = 91.9243%