In 1963, the U.S. Postal Service introduced the ZIP code to help process mail more efficiently. Postal workers sort through bulk mail.

a. A ZIP code contains five digits, 0-9. How many possible ZIP codes are there?
b. In 1983, the U.S. Postal Service introduced ZIP + 4. The extra four digits at the end of the ZIP code help pinpoint the destination of a parcel with greater accuracy and efficiency. How many possible ZIP + 4 codes are there? The Canadian postal service uses a six-character mailing code of the form letter, digit, letter, digit, letter, digit
c. How many possible Canadian postal codes are there if no restrictions are placed on the letters and digits?
d. In Canadian postal codes, the letters D, F, I, O, Q, and U are never used and the letters W and Z are not used as the first characters. How many possible postal codes are there now?