| Summary: | Regula de Tri, the Rule of Three, is in its simplest form a way of computing the fourth proportional in an equation of two ratios. It was an indispensable topic in Iceland from the first textbooks in arithmetic in the 18th century until the 1970s. Instruction in the Rule of Three in compulsory education developed into a ritual procedure for exercises of certain types and content, often without relevance to contemporary life or reference to ratios and proportions. It was subjected to severe criticism at times of changes in compulsory education, in the early 20th century and at the introduction of the “New Math” in the 1960s. At that time the Rule of Three was quashed once and for all, and attention was turned to the plain concepts of ratio and proportions. Peer reviewed
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