Lets take them one by one.
[I am assuming that 2 people (say A and B) are married and both are a part of the 11]
a) The answer is
Case when the couple (group of two) attends + the case when the couple doesn't attend
When the couple attends, you need to choose 3 out of remaining 9. C(9,3)
When they don't, you need to choose 5 out of remaining 9. C(9, 5).
The answer therefore = C(9, 3) + C(9,5) = 84 + 126 = 210.
b) The total ways of inviting = C(11, 5) = 462.
The ways in which these 2 wud not be invited together = total ways of invitation - ways in which they would be invited together.
Now the ways in which they're invited together:
Assume they make 1 group. So ways in which they're invited together = C (9, 3) + C(9, 5) // when they're invited + when they're not invited
Therefore, the ways in which they would not be invited together
= 462 - 210 = 252.
Note that the two being invited together is the same as the "invited together" case of the couple in (a).
Hope this helps.
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