26 letters for each letter
4 letter = 26*26*26*26 = 26^4
5 letter = 26*26*26*26*26 = 26^5
26^4 + 26^5 = 26^4 ( 26+1) = 27*26^4
Hope this helps.
ps9
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Source: Beat The GMAT — Data Sufficiency |












