Since we can choose 2 letters and they can not be the same, we have the following possibilities:
[tex]{}\lbrace AB,AC,AD,BC,BD,BA,CB,CA,CD,DA,DB,DC\rbrace[/tex]As we can note, there are 12 possible combinations of 2 different letters. So, How many such passwords are possible? The answer is 12 passwrods
