If A is to B as C is to D, enter A, B, and C to find D. This is the classic "rule of three" used in everyday calculations: scaling recipes, reading maps, currency exchange, and more.
The rule of three (or cross-multiplication) finds a missing value in a proportion A:B = C:D. Formula: D = B×C/A. Example: if 3 workers finish a job in 4 days, 6 workers finish in D = 4×3/6 = 2 days (inverse proportion).
A/B = C/D ⟹ D = (B × C) / A
Example: 3 apples cost 4€. How much do 9 apples cost? A=3, B=4, C=9 → D = (4×9)/3 = 12€
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