Simple and Compound Interest is a high-scoring arithmetic topic in IBPS PO Prelims, appearing consistently as 2–4 direct questions or embedded within data interpretation sets. The questions test your ability to calculate interest under different compounding scenarios and apply shortcut formulas for quick solutions.
Mastering this topic requires understanding the fundamental difference between SI (interest on principal only) and CI (interest on principal plus accumulated interest). Bank exams favour problems involving 2–3 year periods, half-yearly compounding, and the classic "difference between CI and SI" type. Since these calculations appear frequently in banking operations, expect realistic scenarios involving deposits, loans, and instalments.
Students who memorize the core formulas and practice the standard 2-year and 3-year difference shortcuts can solve most IBPS PO questions in under 60 seconds—making this a reliable marks-fetching area.
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Key Concepts
**Simple Interest (SI)** calculates interest only on the original principal throughout the entire period. The interest remains constant each year.
**Compound Interest (CI)** calculates interest on the principal plus all previously accumulated interest. Each year's interest is higher than the previous year.
**Compounding frequency matters**: Annual compounding means interest is added once per year; half-yearly means twice per year (divide rate by 2, multiply time by 2); quarterly means four times per year.
**For the same principal, rate, and time, CI is always greater than or equal to SI**. They are equal only when time = 1 year with annual compounding.
**The difference between CI and SI for 2 years** equals one year's interest on one year's SI—this creates the shortcut formula.
**Effective rate**: When compounding is more frequent than annual, the effective annual rate exceeds the nominal rate.
**Amount = Principal + Interest** for both SI and CI; this relationship drives most formula manipulations.
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Formulas / Key Facts
**Simple Interest:**
SI = (P × R × T) / 100
Amount (A) = P + SI = P(1 + RT/100)
**Compound Interest:**
Amount = P(1 + R/100)^T
CI = Amount − P = P[(1 + R/100)^T − 1]
**Half-yearly Compounding:**
Amount = P(1 + R/200)^(2T)
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*Note: If compounded annually, CI would be ₹2,000. Half-yearly compounding gives ₹100 more.*
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Common Mistakes
**Confusing SI and CI formulas under time pressure** → Remember: SI has multiplication (P×R×T), CI has power (exponent T). Practice identifying which formula applies before calculating.
**Forgetting to adjust rate and time for half-yearly/quarterly compounding** → Create a mental checklist: Half-yearly = divide R by 2, multiply T by 2. Always write the adjusted values before substituting.
**Calculating CI instead of the difference (CI − SI)** → Read the question twice. If it asks for "difference," use the shortcut formula directly rather than calculating both separately.
**Using the 2-year difference formula for 3-year problems** → The 3-year formula has an extra term: P(R/100)² × (3 + R/100). Memorize both versions distinctly.
**Misreading "compounded annually" as "simple interest"** → Compound interest is the default in bank exam contexts. If it says "per annum" without specifying SI, check for compounding clues.
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Quick Reference
1. SI = PRT/100; CI = P[(1 + R/100)^T − 1]
2. CI − SI (2 years) = P × (R/100)² — fastest shortcut for difference questions
3. Half-yearly: Rate → R/2, Time → 2T
4. CI always ≥ SI; equal only at T = 1 year (annual compounding)
5. To double under SI: Years = 100/Rate
6. For 10% rate: (1.1)² = 1.21, (1.1)³ = 1.331 — memorize these multipliers