A proportion is an equality of two ratios: ab=cd\frac{a}{b}=\frac{c}{d} (or colon notation). Fundamental property of a proportion: the product of the extremes equals the product of the means: a⋅d=b⋅ca\cdot d = b\cdot c. With this rule you can find an unknown term.
For ab=cd\frac{a}{b}=\frac{c}{d}: extremes are a,da,d; means are b,cb,c
Cross products are a quick check for equivalent ratios
Solve for the unknown from the equality of products
Proportion explained
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