改版题求解，谢谢NN们

RichardJing

prep39.        5583-!-item-!-187;#058&004668(NEW Version)

A certain stock exchange designates each stock with a one-, two-, or three-letter code, where each letter is selected from the 26 letters of the alphabet.  If the letters may be repeated but the same letters used in a different order are treated as the same code, how many different stocks is it possible to uniquely designate with these codes?

请问大家，这个题改了后怎么做啊？本人对排列组合不懂，请问这个题有解么？ 下面是原题

39.        5583-!-item-!-187;#058&004668(Original version)

A certain stock exchange designates each stock with a one-, two-, or three-letter code, where each letter is selected from the 26 letters of the alphabet.  If the letters may be repeated and if the same letters used in a different order constitute a different code, how many different stocks is it possible to uniquely designate with these codes? E

(A)  2,951

(B)  8,125

(C)  15,600

(D)  16,302

(E)  18,278

谢谢大家了

RichardJing

没人会做？还是题无解？

RichardJing

谢谢，但是假如 aba 和aab 算是一种code呢，有法做么？

RichardJing

我也是随便一想，太谢谢你了，N，真N，我也在看看