1. The problem statement, all variables and given/known data A word has n letters, in which one of those letters is present a times and another is present b times (all other letters are present only once). a.) How many combinations of x letters are there from this word? b.) How many arrangements of x letters are there from this word? 2. Relevant equations nCr=n!/((n-r)!r!) nPr=n!(n-r)! Any others? 3. The attempt at a solution I'm not completely clear but I think the problem means, e.g. “POSSESSES”, 5 letters being chosen: n=9, r=5, a=5 (5 Ss), b=2 (2 Es). I was told the solution would require a sigma function (summation operator) so it won't be easy. So far, for combinations, I came up with something like Ʃ(i=0 to i=n-a-b; C(n-a-b,i)) but now I'm thinking that's only a small piece of the puzzle. Help?