Intelligence test scores, referred to as intelligence quotient or IQ scores, are based on characteristics such as verbal skills, abstract reasoning power, numerical ability, and spatial visualization. If plotted on a graph, the distribution of IQ scores approximates a normal curve with a mean of about 100. An IQ score above 115 is considered superior. Studies of ‘‘intellectually gifted'' children have generally defined the lower limit of their IQ scores at 140; approximately 1% of the population have IQ scores above this limit. (Based on Biracree, 1984.)

(a) Find the standard deviation of this distribution.

(b) What percent are in the ‘‘superior'' range of 115 or above?

(c) What percent of the population have IQ scores of 70 or below?