I'm revising for finals and I have come across this following question:
a) A woman has 11 close friends. Find the number of ways she can invite 5 of them to dinner.
b) Repeat a) but 2 of the friends are married and will not attend separately.
For a) I got ${11 \choose 5} = \frac{11!}{(11-5)! \cdot 5!} = 462$.
I'm completely lost on b). I tried $n-2$ (minus two friends that are married) and then: ${9 \choose 5} = \frac{9!}{(9-5)! \cdot 5!} = 126$, but I'm pretty sure this is wrong,
Could someone explain b) to me?
Thanks in advance!