Answer
$15,600$
Work Step by Step
There are 26 letters in the alphabet.
The first letter in the code can be any letter of the alphabet =26 choices
The second letter in the code can not be the same as the first letter, but can be any other: 25 choices.
The third letter in the code can not be the same as the first or second letter, but can be any other: 24 choices.
Therefore the number of different codes is $26*25*24=15,600$
15,600 different customers can be identified.
