Correct Answer: X = 11, Y = 14
Whenever a combinatorics question introduces "incompatible" or "cannot be together" rules, it is almost always faster to calculate the Total combinations and subtract the Invalid ones.
The Breakdown
1. Find the Total Possible Committees
We are choosing 3 people from a total pool of 8.
Total = 56 possible committees.
2. Find the Invalid Committees
A committee is invalid if it contains {Aly, Ben} OR {Cam, Dan}.
If Aly and Ben are chosen: They take up 2 of the 3 spots. There is only 1 spot left, which must be filled by one of the remaining 6 candidates. So, there are 6 invalid committees containing Aly and Ben.
If Cam and Dan are chosen: Following the exact same logic, they take up 2 spots, leaving 1 spot for the 6 remaining candidates. There are 6 invalid committees containing Cam and Dan.
(Note: We don't have to worry about double-counting a committee that has Aly, Ben, Cam, and Dan, because a committee only has 3 spots!)
Total Invalid Committees = 6+6=12.
3. Calculate the Probability
Valid Committees = Total − Invalid = 56−12=44.
Probability =
Total
Valid
=
56
44
.
Simplify the fraction by dividing the top and bottom by 4:
56
44
=
14
11
Therefore, X = 11 and Y = 14.