Question 9
A chest checks an entered password from left to right, spending one second per character. It sounds an alarm immediately on the first wrong character. Thus two correct characters followed by a wrong third character cause an alarm after three seconds. The record below has unreadable characters shown by asterisks; these are missing recorded letters, not wildcard inputs.
The eight trials, in order, alarm after 1,8,7,2,4,9,5,10 seconds. Among the first nine password characters, how many different letters occur more than once?
The eight trials, in order, alarm after 1,8,7,2,4,9,5,10 seconds. Among the first nine password characters, how many different letters occur more than once?