Who is asking: Student
The account numbers look like
To count the number of account numbers think about constructing all of them. To start you have 26 choices for the beginning letter. Next, to each such one character account number you can append any of the 10 digits to put in the second place. Hence each of the 26, one character account numbers can be turned into 10, two character account numbers. Thus for the first two places you have 26x10 = 260 choices.
Now that you have the 260 possible two character account numbers in front of you, to each you can append any of the 10 digits as a third character. This will give 260x10 = 2600 three character account numbers.
The pattern is now clear. In total you have