RRB NTPC Preparation Series: Part 3 - Simple & Compound Interest in Maths

RRB NTPC Preparation Series: Part 3 - Simple & Compound Interest in Maths 

Friends! Ee RRB NTPC 2025 preparation series lo Part 3 ki vacham, topic "Simple & Compound Interest" meeda. Idi Maths lo high weightage chapter –

• CBT 1 lo 2-3 questions (7-10% weightage),
• CBT 2 lo 3-4 questions (9-11% weightage),
• Total Maths lo ~20% questions (with Profit & Loss, Percentage combine).

2025 Update (Oct 27): RRB released CEN 07/2025 (UG) today – total vacancies now 11,910 (Graduate + UG). Application UG: Oct 28 to Nov 27. Syllabus same, but interest questions lo GST, EMI, installment variants ekkuva vastunnayi. Exam tentative Jan-Feb 2026.

• Formulas (12+)
• Shortcuts (6+)
• Examples (5 detailed, NTPC style)
• Practice (10 graded questions)
• Integration with Part 1 (Percentage) & Part 2 (Profit & Loss)
• 2025-specific tips

Telugu lo full explanation (English typing lo, simple language), English lo key points & summaries. Peddaga undi, but easy ga read cheyochu – notes teesko, daily practice chey. Next part (Time & Work?) adugithe cheptha. Let’s crack NTPC Maths 30+ marks! 💪

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Step 1: Introduction & NTPC Relevance (Why This Topic?)

Friends, Simple Interest (SI) ante loan or deposit meeda fixed rate tho time basis meeda vastundi. Compound Interest (CI) ante interest on interest – growth faster.

NTPC lo em vastundi?

• Direct: SI/CI calc, difference find.
• Advanced: Installments, EMI, population growth, depreciation.
• 2025 trend: GST on interest, half-yearly compounding, installment loans.

Telugu: Ee topic Profit & Loss tho link undi – interest = profit for bank. Percentages (Part 1) base. CBT 2 lo word problems ekkuva. English: Interest is core arithmetic. High scoring if formulas memorized. Accuracy 100% possible.

Exam	Questions	Marks	Difficulty
CBT 1	2-3	2-3	Easy-Moderate
CBT 2	3-4	3-4	Moderate-Hard

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Step 2: Basic Concepts (Building Blocks)

Friends, basics clear chey – confusion zero.

Term	Meaning (Telugu)	English
Principal (P)	Mooladhanam (konadam amount)	Initial amount
Rate (R)	Vaddi % per year	Interest rate %
Time (T)	Samayam (years lo)	Duration
Amount (A)	Total (P + Interest)	Final value
SI	Simple vaddi	Fixed per year
CI	Chittivaddi	Interest on interest

Telugu: Bank lo 1000 petti 10% rate, 2 years – SI simple, CI fast grow avuthundi. English: SI = linear growth, CI = exponential.

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Step 3: All Key Formulas (12+ with Telugu Explanation)

Friends, memorize chey – ee formulas 90% questions cover chestayi.

#	Formula	Telugu Explanation	Example
1	SI = (P × R × T) / 100	Mooladhanam × rate × time ÷ 100	P=1000, R=10, T=2 → SI=200
2	Amount (SI) = P + SI	Total = P + vaddi	1000 + 200 = 1200
3	CI = P × (1 + R/100)^T - P	Chittivaddi formula	P=1000, R=10, T=2 → CI=210
4	Amount (CI) = P × (1 + R/100)^T	Total amount	1000 × 1.1² = 1210
5	CI - SI (2 years) = P × (R/100)²	2 years difference	1000 × (0.1)² = 10
6	CI - SI (3 years) = P × (R/100)² × (3 + R/100)	3 years diff	Complex, but formula ready
7	Half-yearly CI = P × (1 + R/200)^(2T)	6 months ki compound	R=10 → 5% per half
8	Quarterly CI = P × (1 + R/400)^(4T)	3 months ki	R=12 → 3% per quarter
9	Installment (SI) = Total / (n + interest parts)	EMI la calc	Complex, step-by-step
10	Population Growth = P × (1 + R/100)^T	CI use	1000 people, 10% growth
11	Depreciation = P × (1 - R/100)^T	Value decrease	Machine value drop
12	GST on Interest = Interest × (1 + GST%/100)	Final payable	SI=200, 18% GST → 236

English Summary: SI = linear, CI = exponential. Use (1 + R/100)^T for CI.

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Step 4: Shortcuts & Time-Saving Tricks (NTPC Exam Hacks)

Friends, 90 mins lo 35 questions – shortcuts must!

Shortcut	How to Use	Example
1. Assume P=100	% calc easy	R=10, T=2 → SI=20, CI=21
2. CI for 2 years	P(1+R/100)² - P	Direct
3. Difference CI-SI (2 yrs)	= P × (R/100)²	1000 × 0.01 = 10
4. 1 year CI = SI	Same for 1 year	Obvious
5. Half-yearly	Rate ÷ 2, Time × 2	R=12 → 6%, T=1 → 2 periods
6. Fraction Trick	10% = 0.1, 5% = 1/20	Fast multiply

Telugu: P=100 assume chey – calc 10 secs lo avuthundi. English: Memorize CI table for R=10, T=1,2,3.

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Step 5: Step-by-Step Examples (NTPC Previous & 2025 Style)

Friends, 5 real exam-level examples – step-by-step solved.

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Example 1 (Basic SI - CBT 1) Q: P= Rs. 5000, R=8%, T=3 years. Find SI & Amount. Step 1: SI = (5000 × 8 × 3) / 100 = 1200 Step 2: Amount = 5000 + 1200 = 6200 Telugu: Simple calc, 30 secs lo.

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Example 2 (CI vs SI - CBT 2) Q: P= Rs. 8000, R=10%, T=2 years. Find CI - SI. Step 1: SI = (8000×10×2)/100 = 1600 Step 2: CI = 8000 × (1.1)² - 8000 = 9680 - 8000 = 1680 Step 3: Diff = 1680 - 1600 = 80 Shortcut: P × (R/100)² = 8000 × 0.01 = 80 ✓

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Example 3 (Half-Yearly CI - 2025 Trend) Q: P= Rs. 10000, R=12% p.a. compounded half-yearly, T=1.5 years. Amount? Step 1: Rate per half = 6%, Periods = 3 Step 2: A = 10000 × (1.06)³ = 10000 × 1.191016 = 11910.16 Telugu: Time × 2, Rate ÷ 2 – remember!

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Example 4 (Installment - Advanced) Q: Rs. 1210 paid in 2 equal installments at 10% SI p.a. Find each installment. Step 1: Let installment = x. Total = x + x + interest on first Step 2: Interest on 1st = x × 0.1 × 1 = 0.1x Step 3: 2x + 0.1x = 1210 → 2.1x = 1210 → x = 576.19 English: Use installment formula or equal payment method.

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Example 5 (Depreciation + GST - Tricky 2025) Q: Machine CP= Rs. 1,00,000 depreciates 20% p.a. (CI). After 2 years, sold with 18% GST. Final receipt? Step 1: Value after 2 yrs = 100000 × (0.8)² = 64000 Step 2: GST = 64000 × 0.18 = 11520 Step 3: Total = 64000 + 11520 = 75520 Telugu: Depreciation = CI with negative rate.

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Step 6: Practice Questions (10 Graded - Easy to Hard)

Friends, solve chey – answers + explanation end lo.

Easy (CBT 1 Level)

1. P=2000, R=5%, T=4 yrs. SI?
2. SI=600, P=3000, T=5 yrs. R?

Medium (CBT 2) 3. CI for 2 yrs at 10% on P=5000? 4. Diff between CI & SI for 3 yrs at 10% on P=10000? 5. Half-yearly CI: P=8000, R=8%, T=1.5 yrs. Amount?

Hard (2025 Style) 6. Rs. 1331 paid in 3 equal installments at 10% SI. Each? 7. Population 10000, grows 10% p.a. (CI). After 2 yrs? 8. Machine depreciates 20% p.a. When value = 4096 from 10000? 9. Loan 5000 at 12% SI, paid in 2 installments of 3000 & ?. Find 2nd. 10. P=10000, R=10%, T=3 yrs CI quarterly. Amount?

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Step 7: Answers + Explanations + Tips

Answers:

1. 400
2. 4%
3. 1050
4. 310
5. 8608.64
6. 441
7. 12100
8. After 3 years
9. 2800
10. 13448.96

Tips for NTPC 2025:

• Daily: 20 questions (RS Aggarwal Ch. 12-13).
• Mock: Adda247 2025 series lo interest section focus.
• Revise: CI table for R=5,10,20 & T=1,2,3.
• Avoid: Calculator ledu – fraction multiply practice.
• Link: Interest = Profit for bank → Part 2 revise.