A certain stock exchange designates each stock with a one-, or 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 constitutes a different code, how many different stocks is it possibly to uniquely designate with these codes? A. 2951 B. 8125 C. 15600 D. 16302 E. 18278 Answer : E Please tell me how to do ??? Thankx |