Box Rule
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| 3 | 2 | |||||||||||
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Look at the green box (rectangular area): two 3 must be placed in the box. But the box is too small to place them in the same row.
Therefore each green row must contain a 3.
Therefore neither yellow field can be 3.
| 1 | ||||||||||||
| 3 | 2 | |||||||||||
| 5 | ||||||||||||
| 1 | ||||||||||||
Look at the green box (rectangular area): two 3 must be placed in the box. But the box is too small to place them in the same row.
Therefore each green row must contain a 3.
Therefore neither yellow field can be 3.