Number of different 3 letter words = (# of 3 letter words with two Is) + (# of 3 letter words with two Ns) + (# of 3 letter words with every letter distinct)
# of 3 letter words with two Is: (# of ways to select two Is) * (# of ways to select a 3rd letter) * (# of ways to place that 3rd letter)
# of ways to select two Is = 1
# of ways to select a 3rd letter = 3 (select 1 letter from Z,N,A)
# of ways to place that 3rd letter = 3 (*I*I* --> * marks where we can place the 3rd letter)
# of 3 letter
...
# of 3 letter words with two Is: (# of ways to select two Is) * (# of ways to select a 3rd letter) * (# of ways to place that 3rd letter)
# of ways to select two Is = 1
# of ways to select a 3rd letter = 3 (select 1 letter from Z,N,A)
# of ways to place that 3rd letter = 3 (*I*I* --> * marks where we can place the 3rd letter)
# of 3 letter
...
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