Manbehindthecurtain wrote:

Can you explain why the four and five letter code has 26^4 and 25^5 different permutations? I thought I would have to work out a very complicated permutations formula on this problem in order to figure it out...

Coming to 4 letter code, each letter can be any one of the 26 alphabets. So total number ways of choosing 4 letters will be 26 x 26 x 26 x 26 = 26^4

Similarly 5 letter code we will have = 26^5

Total = 26^5+26^4 = 26^4(26+1) = 27 (26^4)

In case if no two letters of the code are same, then the first letter of the 4 letter code can be chosen in 26 ways, the second letter of the code can be chosen in 25 ways, third letter in 24 ways and the 4 letter in 23 ways. Hence in this case the total number of 4 letter codes will be 26 x 25 x 24 x 23

Hope this is clear

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Averages Accelerated:Guide to solve Averages Quickly(with 10 practice problems)