How it works
Arrange 1–15 in row order, with the empty square at the bottom right. Only a tile directly above, below, left or right of the gap can move. Every puzzle is produced by legal moves from a solved board, so it is solvable.
A useful example
If the last row is 13, 14, empty, 15 while the other rows are solved, tap 15 once. It slides left, leaving the empty square in the final position. That is one move.
Work on the top row first, then build the next row while protecting the completed area. A tile in the right position may need to move temporarily; fewer moves are an optional personal challenge, not a timed requirement.
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