Question 332451
The answer depends on a couple of things

-The language used 
-Whether replication is allowed or not
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Lets assume we are talking about the English language, there are 26 possible letters.

And of course we'll assume we are talking about the numbers 0 thru 9
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With replication
2 letters followed by 2 number:  26*26*10*10 = 67600 codes
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without replication
2 letters followed by 2 number:  26*25*10*9 = 58500 codes
There are 26 choices for the first letter, afterwards there are 25 choices for the second letter, since no replication is allowed.