Problem statement: Of the letters in the word "POSSESSES", if you must choose 5 letters: a) How many possible combinations are there (of 5)? b) How many possible arrangements are there (of 5)? Given a word of n letters, where one of the letters is present 'a' times in the word and the other is present 'b' times in the word, how many possible combinations of r letters are there, and how many possible arrangements of r letters are there? My comments: Actually I don't have a clue where to start with this. Don't worry about finding that general formula; that part of the question is an extension written by me for myself, which I will attempt once I've seen the method. Frankly I was surprised it was this difficult to consider repeated elements in nPr and nCr, but it would really help me if you would outline in detail the method to take to solve such a problem, so I could then try to apply it to a general case. Thanks in advance for any help.