The following questions there're always 26 letters (a-z) and 10 digits (0-9). Identifiers start with letter, followed by letters or digits. Passwords are strings of letters and digits. Where proper, state which rule you employed, show the formula you employed to derive your answer.
1. How many functions are there from letters to digits?
2. How many one to one functions are there from digits to letters?
3. How many three character identifiers are there?
4. How many three character passwords are there with at least one digit?
5. At least how many cards are required so that three are of same suit
6. How many positive integers less than 1000 are divisible by 7?
7. How many positive integers less than 1000 are divisible by 7 or 11?