Ratio is a comparison between two quantities using division. Represented as a:b or a/b. Key concepts: (1) Ratio of a:b means a÷b units; (2) Ratios can be simplified by dividing both by their GCD; (3) Equivalent ratios exist when both parts are multiplied by the same number. Formula: If a:b:c = x:y:z, then a/x = b/y = c/z. Solving MCQs: Identify common ratio and multiply to find values. If a:b = 3:5 and a = 15, then b = 25. Shortcut: Use cross multiplication for quick verification. Common trap: Forgetting to simplify to lowest terms. Exam tip: Always check if ratios need to be in same units (e.g., hours to minutes). Practice expressing word problems as ratios.