Place these 16 numbers, into the proper squares of the 4x4 grid so that...
9, 9, 8, 8, 7, 6, 6, 5, 4, 4, 4, 4, 4, 3, 2, 1
1. Each row of squares adds up to 21.
2. Each column of squares adds up to 21.
3. The four corner squares add up to 21.
4. The four central squares add up to 21.
5. The two squares above and the two squares below the central square add up to 21.
6. The two squares to the right and the two squares to the left of the central square add up to 21.
7. The diagonal squares from upper left to lower right add up to 21.
8. The diagonal squares from upper right to lower left add up to 21.
Please put your solution in a "spoiler box" to hide it from others
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Denis ... yes there are variants