These images are from a 3 page article by Dr. Maria Koth which also has a student template with multiple triangles and another with one large Rechendreieck suitable for demonstration.

Her webpage has more articles and information.
If you don't read German, use Google to translate the pages.
Rechendreieck are calculation triangle puzzles that encourage flexible thinking and
fact fluency.

More links and information will follow on the German approach to fact fluency.