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Solves proportions (equal ratios) to find an unknown value using cross-multiplication. If a/b = c/d, then a×d = b×c.
Proportions were central to Euclid's Elements (c. 300 BCE), where Book V is entirely dedicated to the theory of proportions — considered the foundation of Greek mathematics.
Cross-multiplying before setting up the proportion correctly — make sure corresponding parts are on the same side before solving.