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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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