(password counting) each of the following cases contain rules for setting a password. analyze the number of valid password combinations for each case : (a) passwords of lengths 6 to 8 characters long, where each character is either a lower case letter (a-z) or a digit (0-9) (b) passwords of lengths 6 to 8 characters long containing at least 1 digit, where each character is either a lower case letter (a-z) or a digit (0-9).
(c) passwords of length 8 which contain at least one vowel in them, where each character is a lower case letter. (d) passwords of length 8 which contain only digits (0-9), but no digit can be repeated.