Answer: 9.
- A 9
- B 8
- C 12
- D 24
Correct answer: A. 9
Explanation: Inclusion-exclusion: D(4) = 4!(1−1+1/2!−1/3!+1/4!) = 24(1/2−1/6+1/24) = 24(12/24−4/24+1/24) = 24×9/24 = 9.
Counting tree for selecting 2 items from {A, B, C} without repetition: 3 x 2 = 6 ordered arrangements.
Concept context
Counting, arrangements, and selections