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Because... I do not know :-). Everytime, anybody do these kind of quiz, there is a catch. So I really do not know, why they are not doing more research...

I'll have a look at the image after the server upgrades are done and we can upload images again.

I opted for 24, still curious about the right answer?

I just looked at this picture carefully.

There are 18 triangles in this picture.
It is confirmed.