Given an alphabet of A different letters, how many K length words can we form that have exactly D different letters?
The answer is given here:
Permutations of fixed length words of an arbitrary alphabet with fixed number of different letters
but, with my limited mathematical knowledge of the presented formula "the additive recursion from Marc van Leeuwen" , i see no possibility to calculate my problem.
The example: Alphabet = 18 letters. Permutations for a 3, 4,5, 6 letter word always has at least half of the letters consonants and the other letters are vowels. The vowels can be 4 or another number.