People were polled on how many books they read the previous year. Initial survey results indicate that s = 13.7 books. Complete parts (a) through (d) below. Click the icon to view a partial table of critical values. . . . . . (a) How many subjects are needed to estimate the mean number of books read the previous year within six books with 90% confidence? This 90% confidence level requires subjects. (Round up to the nearest subject.)