1. The problem statement, all variables and given/known data You have 7 birds lined up to feed. Only 3 birds can feed at a time. Two of the 7 birds do not like to feed with each other. How many combinaions can be formed? 2. Relevant equations C(n,k)=n!/k!(n-k)! 3. The attempt at a solution Attempted solution C(7,3)-C(7,2)=21. That is the wrong answer. The correct answer is C(7,3)-C(2,2)*C(5,1)=5 Can someone explain this to me? Thanks. My thinking was 7 choose 3 minus 7 choose the two birds that don't like to mix. Subtract and you'll get the birds that mix. Obviously its wrong -_-. Thanks for your help in advance.