To solve this problem, we make use of the z statistic. A population of 5% means that we are looking for the population at >95%, P = 0.95. Using the standard distribution tables for z, a value of P = 0.95 indicates a value of z of z = 1.645
Now given the z and standard deviation s and the mean u, we can calculate for the value of IQ of the top 5% (x):
x = z s + u
x = 1.645 (15) + 100
x = 24.675 + 100
x = 124.675
Therefore the minimum iq score for the top 5% of the population is around 124.675
