total number of combinations

Hey. I have 3 problems I can't get. If anyone can help with how to do any of them, it would be appreciated.

1) To gain access to his account, a customer using an ATM must enter a four-digit code. If repetition of the same four digits is not allowed (for example, 5555) how many possible combinations are there?

2) Over the years, the state of California has used different combinations of letter of the alphabet and digits on its automobile license plates.

a. At one time license plates were issued that consisted of three letters followed by three digits. How many different license plates can be issued under this arrangement?

b. Later on, license plates were issued that consisted of three digits followed by three letters. How many different license plates can be issued under this arrangement?

3) An exam consists of ten true-or-false questions. In how many ways may the exam be completed if a penalty is imposed for each incorrect answer, so that a student may leave questions unanswered?