Mahesh, Before dealing with that, how did you find 23! permutations not containing "fish"? Thanks. Ben
> Date: Sat, 19 May 2012 02:35:41 -0400 > From: firstname.lastname@example.org > To: email@example.com > Subject: Re: RE: Permutation problems > > I could make some progress after your reply. I thought of doing it other way round. So now the sets are: > A: Contain fish |A|=23! > B: Contain rat |B|=24! > C: Contain bird |C|=23! > A n B: 21! > A n C: 20! > B n C: 21! > A n B n C: 18! > > A' n B' n C'= Total permutations - (A u B u C) > = 26!-(23! + 24! + 23! - 21! - 20! - 21! +18!) > =4.0261.... * 10 ^ 26 > Let me know if this is correct. > Thank you.