The proportion of public accountants who have changed companies within the last three years is to be estimated within 3%. The 95% level of confidence is to be used. A study conducted several years ago revealed that the percentage of public accountants changing companies within three years was 21.

a. To update this study, the files of how many public accountants should be studied?

b. How many public accountants should be contacted if no previous estimates of the population proportion are available?