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Half-Yearly / Quarterly CI

ITBP Constable · Quantitative Aptitude

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Core Concept

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When you deposit money in a bank, the bank usually adds interest once a year. But some banks add interest twice a year (half-yearly) or four times a year (quarterly). Each time interest is added, it becomes part of the new principal, and the next interest is calculated on this larger amount. This is why more frequent compounding gives you more interest

💡Key Rules

For half-yearly CI: The rate is divided by 2, and time is multiplied by 2. For quarterly CI: The rate is divided by 4, and time is multiplied by 4

🔑Formula

A = P × (1 + R/(100×n))^(t×n) Where: - A = Amount after interest - P = Principal (original money) - R = Annual rate of interest (%) - n = Number of times compounded per year (2 for half-yearly, 4 for quarterly) - t = Time in years - CI = A − P

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Exam Patterns

What examiners ask — read before attempting PYQs

SSC CGL typically asks: Compare CI for different compounding periods, find CI amount, or calculate effective rate. Shortcut/Trick: For half-yearly: Use R/2 and 2t. For quarterly: Use R/4 and 4t. Always remember the rate gets divided and time gets multiplied by the same number.

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Worked Example

Solve this step-by-step before moving on

1

Step 1

Identify n = 4 (quarterly)

2

Step 2

Apply formula: A = 8000 × (1 + 20/(100×4))^(1×4)

3

Step 3

A = 8000 × (1 + 5/100)^4

4

Step 4

A = 8000 × (1.05)^4

5

Step 5

A = 8000 × 1.2155 = 9724

6

Step 6

CI = 9724 − 8000 = Rs 1724 Common Mistake: Students forget to divide the rate by the compounding frequency. They use the full annual rate instead of R/2 or R/4, leading to wrong answers. Always reduce the rate first.

🔑 Key Points

Half-yearly CI: Divide rate by 2, multiply time by 2
Quarterly CI: Divide rate by 4, multiply time by 4
Formula: A = P(1 + R/(100n))^(tn) where n = compounding frequency
More frequent compounding = higher final amount
CI = Amount − Principal (always calculate both separately)
In 1 year, quarterly compounding gives more interest than half-yearly

📌 Exam Facts

For half-yearly compounding, the effective rate formula is: (1 + R/200)^2 − 1
For quarterly compounding in 1 year, total compounding periods = 4
Half-yearly means n = 2, so rate becomes R/2 for each period
Quarterly means n = 4, so rate becomes R/4 for each period
If time is 2 years with quarterly compounding, total periods = 8
Compound Interest formula with frequency: A = P(1 + r/100)^n where r is periodic rate and n is total periods
For half-yearly: 1 year = 2 periods, 2 years = 4 periods, 3 years = 6 periods

🚀 60-Second Revision

Remember: Divide rate by compounding frequency (2 for half-yearly, 4 for quarterly), multiply time by the same number
Formula: A = P × (1 + R/(100×n))^(t×n) — this works for ALL compounding frequencies
Trap: Don't forget CI = Amount − Principal; calculate both separately
Quick Check: In 1 year with quarterly CI at 20% p.a., effective rate ≈ 21.55% (not 20%)
Pattern: More frequent compounding always gives MORE interest for same P, R, and t
Always verify: After substitution, ensure exponent = compounding periods per year × time in years

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Test yourself under real exam conditions

A timed ITBP Constable mock shows exactly how Half-Yearly / Quarterly CI questions appear in the actual paper — and where you lose marks.

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