Problem 1. Websites often set requirements on users' passwords strength. For example, a password must be at least 8 characters long, or to have at least one capital letter, etc. Suppose a user wants to set a password consisting of n characters, each can be either a small or capital Latin letter, or a digit; no other symbols are allowed. Find the number of different passwords of length n if (a) a password may consist of any digits and letters in any order; (b) a password must have at least one digit and at least one small or capital letter. (c) a password must consist of only digits in the non-decreasing order