The last digit, d, of a phone number is a function of n, its position in the phone book. Table 2.8 gives d for the first 10 listings in the 2009 New York State telephone direc-tory." The table shows that the last digit of the first listing is 1, the last digit of the second listing is 5, and so on. In
principle we could use a phone book to figure out othe
values of d. For instance, if n = 300, we could count
down to the 300th listing in order to determine d. So we
write d = f(n).