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?
(A) 2,951
(B) 8,125
(C) 15,600
(D) 16,302
(E) 18,278
答案是E. 請問這一題怎麼解呢? 我研究了很久還是不會><. 麻煩大家了.
