[Math] If two members refuse to be on the same team, how many teams are possible

combinatorics

An engineering group consists of 12 men and 13 women.

If 2 women refuse to be on the same team together, how many different project teams can be formed consisting of 5 men and 5 women?

Best Answer

Count the number of ways to choose $5$ men from the $12$ available; I think that you know how to do that. Now call the awkward women $A$ and $B$.

  1. Count the number of ways to choose $5$ women from the $11$ more sociable ones, so that neither $A$ nor $B$ is chosen.
  2. Count the number of ways to choose $A$ and $4$ of the $11$ more sociable women.
  3. Do the same for $B$.

Alternatively, you can count the number of unusable $5$-woman groups, i.e., those that contain both $A$ and $B$, and subtract that from the number of all possible $5$-woman groups.

Now you have the total number of ways in which you can choose the women. Multiply by the number of ways to choose the men, and you’re done.