III.1. What’s the probability? If you have access to a printed copy, open your local Yellow Pages telephone directory to any page in the Business White Pages listing. Look at the last four digits of each telephone number, the digits that specify an individual number within an exchange given by the first three digits. Note the first of these four digits in each of the first 100 telephone numbers on the page.
If you don’t have access to a printed directory, you can use an online directory. Find an online directory with at least 100 different phone numbers. Look at the last four digits of each telephone number, the digits that specify an individual number within an exchange given by the first three digits. Note the first of these four digits in each of the first 100 telephone numbers in the directory.
One directory we found online was at http://education.ohio.gov/Contact/Phone-Directory. State government agencies often have directories with office phone numbers of employees.
(a) How many of the digits are 1, 2, or 3? What is the approximate probability that the first of the four “individual digits’’ in a telephone number is 1, 2, or 3? (Hint: See page 407.)
(b) If all 10 possible digits had the same probability, what would be the probability of getting a 1, 2, or 3? Based on your work in part (a), do you think the first of the four “individual digits’’ in telephone numbers is equally likely to be any of the 10 possible digits? (Hint: See page 429.)