To make one triangle you need 3 dots. There are 12 in total.
Use combinatorics to find how many ways you can join
any 3 dots from the 12, leading to 220.
But, you say, you could also generate straight lines. Subtract them! 4 vertical lines, 4 diagonals and 12 lines in each row,
making a total of 20 straight lines. Therefore, there are 200 triangles in the 3×4 grid.
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