Notes on this puzzle
Fifteen #5212 is a 4×4 board with 15 numbered tiles and one gap, which starts in row 4, column 4. 5 tiles start in their home squares.
Add up how far each tile is from its home square and you get 18, so no solution can be shorter than 18 moves. The shortest possible solution is exactly 24 moves. Matching it is a real achievement. Solving it row by row, the method from our guide, takes 40 moves on this board. Tile 12 has the longest trip home: 3 squares.
Where to beginTiles are not far from home after this short shuffle; fix one corner, then grow outwards from it.