A free original bilingual lesson for the Ratios Percentages and Commercial Comparisons module of RRB Group D Mathematics. Identify the changing quantities and fixed conditions before selecting a model. Use unit rates, scale factors and constant products; convert units; and reject examples whose quotient or product is not constant. Worker, stock and map examples state their required assumptions. Entry checks, coached examples, an analogy, a quick reference and 12 practice questions with explained answers support learning. This PDF covers this topic only, not the complete Mathematics syllabus or a full CBT simulation.