A free original bilingual teaching lesson that follows the existing Simple Interest lesson in RRB Group D Mathematics. Build a period-by-period ledger, derive the compound-interest formula and compare simple and compound interest under stated conditions. Match rates with annual, half-yearly and quarterly periods; distinguish a nominal annual quote from a rate already given per period; and recognise when an incomplete final period needs an explicit rule. Ten worked examples also cover hypothetical growth, depreciation and basic reverse calculations for principal, rate and whole periods. Includes entry checks, an analogy, a quick reference and 16 original practice questions with full solutions and every-option explanations. This PDF covers one interest topic, not the complete Mathematics syllabus or a full CBT simulation. The examples are learning models, not advice about financial products.