+78 How many 4-letter "words: are there in which no letter appears 3 or more times in a row?
+129850 Possible 4 letter words = 26^4
Possible 4 letter words with all letters the same = 26
Possible 4 letter words having three repeated letters = (choose any letter to be repeated 3 times) * choose any of the other 25 letters * let this last letter occupy either the first or last position = 26 * 25 * 2
So...possible 4 letter words where no word has a letter repeated 3 or more times =
26^4 - 26 - 26 * 25 * 2 =
455,460
