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 #3
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0
 #4
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+13
Dec 13, 2018
 #3
avatar+6251 
+1

The way to look at this semi formally is to consider the different casesThere can be 0-2 pairs of letters chosenwith 0 pairs there are (74)4!=840 unique 4 letter wordswith 1 pair there are 3(42)(62)2!=540 unique 4 letter wordsThis last bit deserves some discussionThe 3 comes from the fact that 3 of the letters can be chosen as a pair, I, N, VThe (42) comes from the fact that we select two slots from the 4 to place this pairThe (62) comes from the fact that we can select 2 letters from 6 to complete the wordThe 6 comes from removing the numbers that are the pair, and the 2nd copies of the two other numbersFinally the 2! comes from the fact the these last two letters are distinct and their order matters

 

Finally with 2 pairs there are (32)(42)=18This is choosing two pairs from the 3 available and choosing 2 slots from the 4adding all these up we get840+540+18=1398

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Dec 13, 2018
 #3
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Dec 13, 2018

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