People were polled on how many books they read the previous year. Initial survey results indicate that s = 15.3 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 four books with 90% confidence? This 90% confidence level requires 40 subjects. (Round up to the nearest subject.) (b) How many subjects are needed to estimate the mean number of books read the previous year within two books with 90% confidence? This 90% confidence level requires subjects. (Round up to the nearest subject.)