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The 15 Puzzle
the swap that can't be done
The 15 puzzle has tiles numbered 1 to 15 in a 4 by 4 frame with 1 empty space; you slide tiles into the space. Suppose tiles 14 and 15 start swapped and everything else is in order. Can you slide the tiles back into order? In 1879 two mathematicians published the answer, as the puzzle was becoming a worldwide craze.
Source: A classic puzzle, retold by INFINITE SUM and fact-checked against our curated notes
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