问道prep ps1，算不出选项的答案

newport · 发表于 2010-9-13 12:40:04

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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
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【答案】E
【思路】題目有前提If the letters may be repeated and if the same letters used in a different order constitute a different code,
所以one letter-26種
two letters-26x26種
three letters-26x26x26種
total=18,278
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26^3 =17576,??

问道prep ps1，算不出选项的答案

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