"How many ways are there to arrange the letters in the word 'MISSISSIPPI' in such a way that there are no three consonants in a row?"
I am thinking like this. The following are 'slots' for the letters of our word: _ _ _ _ _ _ _ _ _ _ _. There are 21 consonants in total (English). The first slot as 21 possibilities, the second also has 21, but the third has only 5 possibilities. So how many ways can I place vowels within this word… I don't know.
I can try to enumerate all of the place the I's can go, but there has to be a better way than doing that.
Best Answer
MISSISSIPPI has seven consonants and four vowels (which all happen to be I). Write X for any consonant, and first think of how many possible patterns of the type IXXIXIXXIXX, with seven Xes and four Is exist, with no three Xs in a row. Then think about how to assign the letters MSSSSPP to the Xs. Note that the two subproblems are quite similar to each other. The final step is a simple multiply.
Edit: A bit more on the first subproblem. You'll have to do a bit of hand counting here, I think. There are only four Is, so consecutive Xs can form at most five groups. But there must be at least four groups of Xs, since no group can have more than two Xs. So the configuration of Xs is either XX XX X X X or permutation thereof, of XX XX XX X or permutations thereof. It should be easy to count the permutations in each case. In the first, case, the Is must go one between each group, but in the second, three Is are needed to fill the gaps, and the fourth I can be placed next to one of the other three, or at the beginning or end – five possibilities there.