In how many different ways can four books A, B, C and D be arranged one above another in a vertical order such that the books A and B are never in continuous position?
Correct answer: B. 12
- A. 9
- B. 12
- C. 14
- D. 18
Explanation
There are 4! = 24 total arrangements. If A and B are together, treat them as one block, giving 3! x 2! = 12 arrangements, so the arrangements in which they are not together are 24 - 12 = 12. The likely mistake is 14, which does not correctly count the adjacent cases.
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