Calculate combinations of characters My first post here...not really a math expert, but certainly enjoy the challenge.
I working writing a random string generator and would like to know how to calculate how many possible combinations there are for a particular combination.
I am generating a string of 2numbers followed by 2 letters (lowercase) e.g. 12ab
I think the calculation would be (breaking it down) 
number combinations 10*10=100
letter combinations 26*26=676
So the number of possible combinations is 100*676=67600, but this seems a lot to me so I'm thinking I am off on my calculations!!
Could someone please point me in the right direction?
Thx

Some nomenclature: when you say "2 number", you really mean "2 digits". Also, you need to specify if the digits can be anything or not (for example, do you allow leading zeroes?). 
If each of the two digits can be anything, 0 through 9, and each of the letters can be anything, a through z, then your computation is correct. If you think about it, you can see why the number is not off: any particular pair of letters have 100 numbers that can go before them to make the string. Each particular letter going first has 26 possible "second letters", and each of them has 100 possible pairs of digits to go. So there are already 2600 possible strings of the form xxxa. Another batch for xxxb, etc. They add up very quickly. 
