ENGLISH VERSION
TNPSC Group IV – Simple Interest: Complete Notes and 10 Worked Examples
1. What Is Simple Interest?
Simple Interest is interest calculated only on the original principal throughout the investment or borrowing period.
Previously earned interest is not added to the principal for calculating further interest.
Important terms:
- Principal (P): Original amount invested or borrowed. - Interest (I): Additional money earned or paid. - Rate (R): Interest rate, usually expressed per year. - Time (T): Duration, generally in years. - Amount (A): Principal plus interest.
2. Important Formulas
Simple Interest:
I = (P × R × T)/100
Total Amount:
A = P + I
Principal:
P = (100 × I)/(R × T)
Interest Rate:
R = (100 × I)/(P × T)
Time:
T = (100 × I)/(P × R)
Principal from Total Amount:
P = (100 × A)/(100 + R × T)
3. Time Conversion
- 6 months = 1/2 year. - 9 months = 3/4 year. - 15 months = 1.25 years. - 2 years 3 months = 2.25 years.
For day-based problems, use 365 days in a year unless the problem specifies a different convention.
4. Properties of Simple Interest
1. Annual interest remains constant if the principal and rate remain unchanged. 2. Doubling the time doubles the interest. 3. Doubling the principal doubles the interest. 4. Doubling the rate doubles the interest. 5. Simple interest does not generate additional interest on previously earned interest.
5. Examination Shortcuts
One-year interest = P × R / 100.
One-month interest = P × R / 1200.
Time to double = 100/R years.
Time to triple = 200/R years.
Interest = Total Amount − Principal.
6. Ten Fully Worked Examples
Example 1 – Simple Interest and Total Amount
Find the interest on ₹12,000 at 8% per annum for three years.
I = (12000 × 8 × 3)/100
= ₹2,880.
A = 12000 + 2880
= ₹14,880.
Answer: Interest ₹2,880; Amount ₹14,880.
Example 2 – Time in Months
Find the interest on ₹8,500 at 6% per annum for nine months.
T = 9/12 year.
I = (8500 × 6 × 9)/(100 × 12)
= ₹382.50.
Amount = ₹8,882.50.
Answer: ₹382.50.
Example 3 – Finding the Principal
An investment earns ₹1,536 interest in three years at 8% per annum.
P = (1536 × 100)/(8 × 3)
= 153600/24
= ₹6,400.
Answer: ₹6,400.
Example 4 – Finding the Rate
₹15,000 earns ₹4,500 simple interest in five years.
R = (4500 × 100)/(15000 × 5)
= 450000/75000
= 6%.
Answer: 6% per annum.
Example 5 – Finding the Time
How long will ₹18,000 take to earn ₹2,700 at 5% per annum?
T = (2700 × 100)/(18000 × 5)
= 270000/90000
= 3 years.
Answer: 3 years.
Example 6 – Interest from the Total Amount
Principal = ₹25,000.
Amount after two years = ₹29,500.
I = A − P
= 29500 − 25000
= ₹4,500.
Verification at 9%:
25000 × 9 × 2 / 100 = ₹4,500.
Answer: ₹4,500.
Example 7 – Finding Principal and Rate from Two Amounts
A sum becomes ₹12,400 after two years and ₹16,000 after five years.
Interest for three years:
16000 − 12400 = ₹3,600.
Interest per year:
3600/3 = ₹1,200.
Interest for two years:
2 × 1200 = ₹2,400.
Principal:
12400 − 2400 = ₹10,000.
Annual rate:
1200 × 100 / 10000 = 12%.
Answer: Principal ₹10,000; Rate 12% per annum.
Example 8 – Doubling and Tripling
A sum doubles in eight years under simple interest.
Rate:
R = 100/8 = 12.5%.
Time to triple:
T = 200/12.5 = 16 years.
Answer: 12.5%; 16 years.
Example 9 – Interest for a Number of Days
Find the interest on ₹7,300 at 10% per annum for 73 days. Assume 365 days per year.
T = 73/365 = 1/5 year.
I = (7300 × 10 × 1/5)/100
= ₹146.
Answer: ₹146.
Example 10 – Comparing Investments
First investment: ₹10,000 at 6% for two years.
Second investment: ₹8,000 at 7.5% for two years.
First interest:
10000 × 6 × 2 / 100 = ₹1,200.
Second interest:
8000 × 7.5 × 2 / 100 = ₹1,200.
Answer: Both investments earn the same interest of ₹1,200.
7. Common Mistakes
- Forgetting to convert months into years. - Confusing interest with the total amount. - Applying the compound interest method to a simple interest question. - Confusing annual and monthly interest rates. - Omitting currency units from the final answer.
8. One-Minute Revision
Simple Interest = PRT/100.
Amount = P + I.
Principal = 100I/(RT).
Rate = 100I/(PT).
Time = 100I/(PR).
Time to double = 100/R.
Time to triple = 200/R.
9. Textbook and Book-Back Progress
Textbook: Tamil Nadu Samacheer Kalvi, Class 7 Mathematics, Term III.
Chapter: 2 – Percentage and Simple Interest.
Section: 2.3 – Simple Interest.
Exercise: 2.4.
The previous inventory based on the revised 2022 textbook identified 11 main questions in Exercise 2.4.
- Original textbook questions completed: 0/11. - Pending: 11/11. - Batch 1/3: Questions 1–4. - Batch 2/3: Questions 5–8. - Batch 3/3: Questions 9–11.
2026–27 edition completeness: Source not yet verified.
All ten worked examples are original instructional models, not verified TNPSC previous-year or book-back questions.
10. Spaced Revision Schedule
23 October 2026 – 1-day revision.
25 October 2026 – 3-day revision.
29 October 2026 – 7-day revision.
5 November 2026 – 14-day revision.
11. ETN Push Notification
Title: TNPSC Group IV | Simple Interest – Day 10!
Message: Master simple interest formulas, principal, rate, time, monthly calculations and ten fully worked examples in today's 4 PM Aptitude lesson on ETN News!