Question 6
An ATM password at Fred’s Bank is composed of four digits from 0 to 9, with repeated digits allowable. If no password may begin with the sequence 9, 1, 1, then how many passwords are possible?
**Editorial note for this self-study exercise:** Treat a password as a string of four digits, so leading zeroes are allowed. Only the first three positions forming 911 are forbidden.
**Editorial note for this self-study exercise:** Treat a password as a string of four digits, so leading zeroes are allowed. Only the first three positions forming 911 are forbidden.