Interest is the price of money over time. When you lend money you earn interest; when you borrow money you pay it. "Simple and Compound Interest" (সরল ও চক্রবৃদ্ধি সুদ) is one of the most reliably tested arithmetic topics in Bangladeshi government job exams — BCS Preliminary, all Bank recruitment (Bangladesh Bank AD, Combined/BSCS Senior Officer & Officer, Sonali, Rupali), and IT-officer written papers almost always carry 1–3 marks from it. The good news: the entire topic runs on a handful of formulas and one core idea. Master that idea and you can solve every variant — from a plain "find the interest" question to tricky "the sum doubles / the rates differ each year / installment" problems — often in under a minute.
Learning Objectives
- Define principal, rate, time, interest and amount precisely, and understand what each represents.
- Derive and apply the simple interest formula and rearrange it to find any missing quantity.
- Understand compounding intuitively and apply the compound interest (amount) formula, including non-annual compounding.
- Compare simple vs compound interest numerically and know exactly when they differ.
- Handle standard exam variants: doubling/tripling of a sum, difference between CI and SI, equal-installment repayment, and changing yearly rates.
- Use shortcut formulas safely and avoid the classic traps (unit of time, percentage-of-what, per-period rate).