All of the stocks on the over-the-counter market are designated a 4-letter or 5-letter code that is created by using the 26 letters of the alphabet. Which of the following gives the maximum number of different stocks that can be designated with these codes?

The key is: 27(26^4)

My solution is C(26 4)P(4 4)+C(26 5)P(5 5), but the answer is totally different with the key. Why??? BTW, I find that some problems themselves in the GMATprep are absolutely wrong(such as incorrect meaning of the problem), do anybody notice that?

Thank you guys for help in advance!!!

用4个字母排是26^4,5个字母是26^5，所以是26^4+26^5＝27*26^4

LZ是忽略了字母可以相同的情况，比如AAAA这样排也可以的。