(adsbygoogle = window.adsbygoogle || []).push({}); 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.

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# Homework Help: Simple Combination Help. Confused!

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