In mathematics, in how many ways can 5 books be arranged on a shelf if two specific books must always stand next to each other?

Answer and explanation

Correct answer: D. 48

  1. Step 1: Treat the two adjacent books as a single block: there are ways to arrange the blocks, and internal orderings for the pair, giving . Answer: 48. Why not the others: 24, 120 and 60 do not satisfy the equation or conditions.

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