A certain stock exchange designates each stock with a 1, 2, or 3 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?  Options:
A) 2,951
B) 8,125
C) 15,600
D) 16,302
E) 18,278
if each letter is the same: 26 different combinations   2 letters the same 26^2   all different 26^3   26^3 + 26^2 + 26 = 18278   Answer 18,278.
<result>E</result>
A certain stock exchange designates each stock with a 1, 2, or 3 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?