Proportion is equality of two ratios: a:b = c:d or a/b = c/d. Four terms: a (first), b (second), c (third), d (fourth/consequent). Key property: Product of extremes = Product of means, i.e., a×d = b×c. Continued proportion: a:b = b:c means b² = a×c. Three numbers in GP follow this. Solving: If 4:5 = x:15, then 4×15 = 5×x, so x = 12. Shortcut: Cross multiply immediately for unknowns. Common trap: Confusing proportion with ratio. Exam tip: Practice identifying which term is unknown in a proportion problem. Use product of means/extremes property for verification.