Ratios for recipes, mixtures and maps
A ratio compares two quantities of the same kind. 3:2 means '3 parts of one thing for every 2 parts of the other'. Multiplying or dividing both sides by the same number gives an equivalent ratio.
Solving an equivalent ratio (A:B = C:D) is just cross-multiplication: A·D = B·C. Useful for scaling a recipe, reading a map, or mixing paint.