1. How many Social Security numbers are available if the only restriction is that the number 000-00-0000 cannot be assigned?
2. In 1923, the Federal Communications Commission directed that all new radio stations east of the Mississippi River have call letters beginning with the letter W. How many different three- or four-letter call letters are possible?
3. Before 1995, three-digit area codes for the United
States had the following restrictions:
(i) Neither 0 nor 1 could be used as the first digit.
(ii) 0 or 1 had to be used for the second digit.
(iii) There were no restrictions on the third digit.
How many different area codes were possible?
4. Refer to Exercise 21. Beginning in 1995, restriction (ii) was lifted and any digit could be used in the second position. How many different area codes were then possible?