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Six people A, B, C, D, E and F are to be arranged around a round table. In how many distinct seating arrangements can this be done if A must have either B or C immediately to his right, and B must have either C or D immediately to his right?
- 36
- 12
- 24
- 18
Correct answer: 18
Solution
Fix A. If B sits to A's right, B's right must be C or D -> 2 choices, remaining 3 seats fill in 3! = 6 ways each, giving 12. If C sits to A's right, B's right must be D (C is unavailable), giving a BD block among the 4 remaining seats: 3 positions * 2! = 6. Total 12+6 = 18.
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