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Paint by Three (529)
Other names: Paint by Threes
Instruction:
Fill in each cell to the left of (and above) the main grid with a number, such that each shaded cell contains a number divisible by 3 and each unshaded cell contains a number not divisible by 3. No cell will contain a zero (0).
Then, blacken some cells in the grid such that the numbers to the left (and top) of the main grid represent the lengths of contiguous blackened cell blocks in the corresponding row (or column). The lengths are given in order from left to right (or top to bottom), and cell blocks must contain at least one unblackened cell between them.