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Stock Exchange
A certain stock exchange designates each stock with one-, two-, three- letter code, where each letter is selected from the 25 letters of the alphabet. If the letters may be repeated and if the same letters used in different order constitute a different code, how many different stocks is it possible to uniquely designate with these codes?
a. 2951
b. 8125
c. 15600
d. 16302
e. 18278
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