Pairing gloves within one

You have 55 pairs of gloves labelled with the numbers 11 through 55 (so ten gloves: two of each label). The ten gloves are shuffled and randomly matched into 55 new pairs. What is the probability that every new pair consists of two gloves whose labels differ by at most 11? The probability is 1M\tfrac1M for an integer MM; find MM.

Show hints (2)+
  1. Total matchings of 1010 gloves into 55 pairs is 9!!=9459!! = 945.
  2. Valid pairs join equal or adjacent labels. Count consistent matchings along the label chain 1122334455 (it comes to 2121).

Answer

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45

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Asked at: Jane Street, Two Sigma

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