Suppose we want to study the errors found in the performance of a simple task. We ask a large number of judges to report the number of people seen entering a major department store in one morning. Some judges will miss some people, and some will count others twice, so we don’t expect everyone to agree. Suppose we find that the mean number of shoppers reported is 975 with a standard deviation of 15. Assume that the distribution of counts is normal.

a. What percentage of the counts will lie between 960 and 990?

b. What percentage of the counts will lie below 975?

c. What percentage of the counts will lie below 990?