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Involves polygon sides.
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I just saw that each time for Ma and Tr the number of sides went down by one but Ix stayed the same.
The way I found it was by subtracting the amount of sides in the shape in the 1st and 2nd columns.
E. In every row, the sides of the left polygon is equal to the sides of the remaining two polygons.
Alternative solution, (sequence): The total polygon sides in first row is 16, second row 14, and third row 12.