assume that a password to log on to a computer is 3 letters followed by 2 digits. Determine the number of possible passwords in case that repetation is permitted, and in case it's not primmitted

Question

dumbcow · Answer

with repetition:
each letter has 26 possibilities
each digit has 10 possibilities
=26*26*26*10*10

without repetition:
first letter 26
2nd letter now only 25 
3rd letter only 24
1st digit has 10 choices
2nd digit only 9
=26*25*24*10*9