Will they fit? – 3

I have a Puzzle consisting of 10 × 10 = 100 10 \times 10 = 100 pieces.

What is the minimum number of pieces I have to draw randomly from a pile to be guaranteed that there is at least one 3 × 3 3 \times 3 square (in the solved puzzle) whose 9 9 pieces all have been drawn?

Details and Assumptions:

  • Every piece is equally likely to be picked. I don't look whether the first piece has a straight edge or similar tricks.
  • Every piece only fits together with its neighboring pieces, and there is a unique solution to the puzzle.
  • Two pieces that only share a corner are not joined
  • All 100 pieces of the puzzle are in the pile, but no other pieces


The answer is 92.

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1 solution

Otto Bretscher
Nov 8, 2018

91 is not enough: if all the nine 3i-3j pieces are missing (labeling the pieces like the entries of a matrix), then each 3 × 3 3\times 3 square will miss exactly one piece. But 92 suffices: Consider nine disjoint 3 × 3 3\times 3 squares; at least one of them will be complete.

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