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There are only 4 triangles in this picture. Three are on top and one big triangle.
Triangle have 3 sides and 3 corners.
I don't know how people are saying 9 , 15 and 21.

What @ziabutt3836 said. I counted 18 triangles after I figured out how to access the image.

Too many, I would need quite a patience to count them. I would say 15.

This one's a brain teaser for sure! After counting all the individual triangles and combinations within the larger shape, the total number of triangles is 24.

I just looked at this picture carefully.

There are 18 triangles in this picture.

I just looked at this picture carefully.

Nine obviously.

It looks 9 triangles.

9 triangles I saw.