In the RBI Assistant Mains exam, where every second and every mark matters, Simple and Compound Interest (SI & CI) stand out as one of the most reliable scoring areas in Quantitative Aptitude. Unlike lengthy DI sets or tricky puzzles, SI & CI questions are formula-based, predictable, and quick to solve, making them a perfect opportunity to secure easy marks with high accuracy.
Mastering SI & CI means you can comfortably secure 2–3 direct marks in under a minute per question, without complex calculations or logical traps. In a high-pressure exam like RBI Assistant Mains 2026, this kind of accuracy and speed creates a massive advantage over competitors who spend too long on tougher arithmetic sets.
With the exam scheduled for June 7, 2026, your final selection depends entirely on your performance in this single written test. The Quant section typically includes 12–16 arithmetic questions, and among them, SI & CI consistently appear every year as a reliable 2–3 mark segment.
In short, SI & CI may give you the concept advantage, but mock practice converts that advantage into actual marks in the exam hall.
Download Free PDF — 50 SI & CI Questions for RBI Assistant Mains 2026
What Exactly Is Simple Interest — In Plain Terms
You lend someone money. They agree to pay you back with a little extra — that extra is the interest. Simple Interest works on one principle: the interest is always calculated on the original amount you lent, not on any accumulated interest. Every year, the interest is the same fixed amount. It never grows on itself.
The Formula:
SI = (P × R × T) / 100 Amount (A) = P + SI
Where P = Principal (original sum), R = Rate of interest per annum (%), T = Time in years.
Quick Example 1: Arun invests ₹8,000 at 9% per annum simple interest for 4 years. SI = (8000 × 9 × 4) / 100 = ₹2,880 Amount = ₹8,000 + ₹2,880 = ₹10,880
Quick Example 2: A bank offers SI at 12% per annum. Priya earned ₹3,600 as interest in 3 years. What did she invest? 3600 = (P × 12 × 3) / 100 → P = 360000 / 36 = ₹10,000
What Exactly Is Compound Interest — In Plain Terms
Compound Interest works differently. After each compounding period, the interest earned gets added to the principal — and the next period’s interest is calculated on this new, larger amount. In other words, your interest earns interest too. This is why CI always gives a larger return than SI (at the same rate and time), and why almost all banks and financial institutions use compound interest for loans and deposits.
The Formula:
A = P × (1 + R/100)ⁿ CI = A − P
Where n = number of compounding periods (years for annual compounding).
Key Shortcut — Difference between CI and SI for 2 years:
Difference = P × (R/100)²
This shortcut alone can save you 45 seconds on a common exam question type.
Quick Example 1: Ramesh deposits ₹15,000 at 10% per annum CI, compounded annually, for 2 years. A = 15,000 × (1.10)² = 15,000 × 1.21 = ₹18,150 CI = 18,150 − 15,000 = ₹3,150
Quick Example 2: What is the CI − SI difference on ₹12,000 at 8% for 2 years? Difference = 12,000 × (8/100)² = 12,000 × 0.0064 = ₹76.80 (No need to calculate CI and SI separately — this formula gives the answer directly.)
The SI & CI Topic Breakdown in RBI Assistant Mains 2026
Before jumping into questions, it helps to know what types of word problems actually appear. Based on previous year exam analysis and the 2026 pattern, SI & CI questions in RBI Assistant Mains fall into these categories:
| Question Type | Frequency in Mains | Difficulty |
| Finding SI or CI Amount | Common | Easy |
| Finding Principal (reverse calculation) | Common | Easy–Moderate |
| Finding Rate of Interest | Common | Moderate |
| Finding Time Period | Occasional | Moderate |
| Difference between CI and SI | Very Common | Moderate |
| Half-yearly / Quarterly Compounding | Occasional | Moderate–High |
| Installment-based Problems | Rare (Mains) | High |
| Population Growth / Depreciation (CI-based) | Occasional | Moderate |
| Mixed SI & CI (two-part word problems) | Occasional | High |
The first five types account for the vast majority of what actually appears in the exam. Master those, and you have the topic effectively covered.
Why Mock Practice is Crucial for SI & CI:
- Builds real exam speed: You learn to solve questions in 45–60 seconds under timed conditions.
- Improves accuracy: Repeated exposure reduces silly calculation mistakes in pressure situations.
- Strengthens concepts: Every mock exposes new variations of SI & CI problems.
- Develops exam mindset: You learn which questions to attempt first for maximum score efficiency.
- Boosts confidence: Practicing exam-level questions repeatedly removes fear of Quant.
5 SI & CI Word Problems — Solved with Explanation
Attempt each question on your own before reading the solution. Time yourself: your target is under 75 seconds per question.
Question 1 — Finding Simple Interest (Direct Application)
A shopkeeper borrows ₹24,000 from a cooperative bank at a simple interest rate of 7.5% per annum. He repays the full amount after 4 years. What is the total interest he pays?
(A) ₹6,800 (B) ₹7,200 (C) ₹7,400 (D) ₹7,000 (E) ₹6,600
Answer: (B) ₹7,200
Solution: SI = (P × R × T) / 100 SI = (24,000 × 7.5 × 4) / 100 SI = (24,000 × 30) / 100 SI = 720,000 / 100 SI = ₹7,200
Exam tip: When the rate has a decimal (7.5%), multiply first and divide at the end. Avoid converting to fractions mid-calculation — it costs time.
Question 2 — Finding Principal from Compound Interest
The compound interest on a certain sum at 20% per annum for 2 years is ₹2,640. Find the principal.
(A) ₹5,000 (B) ₹6,000 (C) ₹6,500 (D) ₹7,200 (E) ₹5,500
Answer: (B) ₹6,000
Solution: CI = P × [(1 + R/100)ⁿ − 1] 2,640 = P × [(1.20)² − 1] 2,640 = P × [1.44 − 1] 2,640 = P × 0.44 P = 2,640 / 0.44 P = ₹6,000
Exam tip: (1.20)² = 1.44 is a standard value worth memorizing. Similarly, (1.10)² = 1.21, (1.05)² = 1.1025. These come up constantly.
Question 3 — Difference between CI and SI (Shortcut Application)
The difference between compound interest and simple interest on a sum of ₹18,000 at 10% per annum for 2 years is:
(A) ₹160 (B) ₹180 (C) ₹200 (D) ₹220 (E) ₹250
Answer: (B) ₹180
Solution (using shortcut): Difference (2 years) = P × (R/100)² = 18,000 × (10/100)² = 18,000 × (0.10)² = 18,000 × 0.01 = ₹180
This takes under 20 seconds. If you try to calculate CI and SI separately, it takes 90 seconds. The shortcut is not optional — it is essential for Mains.
Question 4 — Half-Yearly Compounding (Common Trap Question)
₹25,000 is invested at 8% per annum compound interest, compounded half-yearly. What is the amount at the end of 1 year?
(A) ₹27,000 (B) ₹27,040 (C) ₹26,800 (D) ₹27,200 (E) ₹26,500
Answer: (B) ₹27,040
Solution: Half-yearly compounding: Rate becomes 8/2 = 4%, Time becomes 1 × 2 = 2 periods. A = P × (1 + R/100)ⁿ A = 25,000 × (1 + 4/100)² A = 25,000 × (1.04)² A = 25,000 × 1.0816 A = ₹27,040
The trap here is using 8% directly. In half-yearly compounding, always halve the rate and double the periods. This is the most commonly missed step by aspirants.
Question 5 — Mixed Word Problem (Finding Rate)
A sum of money doubles itself at simple interest in 8 years. At what rate per annum is the interest being charged?
(A) 10% (B) 11.5% (C) 12% (D) 12.5% (E) 15%
Answer: (D) 12.5%
Solution: If a sum P doubles, the interest earned = P (since Amount = 2P, so SI = 2P − P = P). SI = (P × R × T) / 100 P = (P × R × 8) / 100 100 = 8R R = 12.5%
The key insight: “doubles itself” means Amount = 2P, so Interest = P. Once you see that, the rest is straightforward substitution.
Download the Free PDF — All 50 SI & CI Word Problems
You have just solved 5 questions. The remaining 45 questions are available in the free PDF below, organized by type and difficulty level — exactly the way they are likely to appear in RBI Assistant Mains 2026.
Conclusion
The RBI Assistant merit list is brutally tight. In competitive cycles, the difference between selection and non-selection has been as small as 1–2 marks. SI & CI questions are among the very few areas in the Quant section where you can walk in with near-certainty of getting full marks — provided you have actually practised the word problem formats.
The candidates who clear RBI Assistant Mains are not necessarily the most intelligent people in the room. They are the most prepared. They have seen enough question variations that nothing surprises them. They have practised enough that their formula recall is automatic, not effortful. And on exam day, they collect 2–3 guaranteed marks from SI & CI in under 3 minutes total while others are still re-reading the question.
Download the PDF. Solve all 50. Analyse your errors. And then attempt a full PracticeMock RBI Assistant Mains mock test — because isolated topic practice only becomes exam performance when you practise it inside a full, timed simulation.
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