1 immediately before 2

Shuffle the numbers 11 through 55 into a uniformly random order. What is the probability that 11 appears immediately before 22 (that is, 22 sits in the very next position after 11)? (to 3 decimals)

Show hints (2)+
  1. Glue '12' into a single block and count arrangements.
  2. Favorable = 4! block orderings, over 5! total.

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Asked at: Timed Mental-Math & Sequences, Game-Based Aptitude

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