Is there any formula that can calculate how many 4 letter codes can be created using only the 26 letters of the Alphabet? For example, a code would...

We are asked
to find the number of 4 letter "codes" that can be created using the 26 letters of the
alphabet.

There are a couple of considerations. Are we allowed to repeat
letters (e.g., is AAAA a viable code?). Also, does order matter (e.g., is ABCD different from
ACBD?) Next, do we differentiate between upper and lower case letters? Since you indicated 26
letters, we will assume that all letters are upper case and that order matters.

1. If you are not allowed to repeat letters, we can use either the multiplication
principle or the permutations formula. A permutation is a way, especially one of several
possible variations, in which a...

href="http://mathworld.wolfram.com/Permutation.html">http://mathworld.wolfram.com/Permutation.html