Study Material — 1 PYQs (2020–2020) · Concept Notes · Shortcuts
RRB Group D Difference SI vs CI is a frequently tested subtopic — 1 previous year questions from 2020–2020 papers are included below with concept notes, key rules and shortcut tricks.
RRB Group D Difference SI vs CI — Past Exam Questions
1 questions from actual RRB Group D papers · all shown free · click option to reveal solution
Exam Q 12020Previous Year Pattern
A sum of money is invested at 10% per annum. The difference between compound interest and simple interest for 2 years is ₹40. Find the principal amount.
Concept Notes
Difference SI vs CI— Rules & Concept
Core ConceptRead this first — the foundation of the topic
The difference between Simple Interest (SI) and Compound Interest (CI) is one of the most frequently tested concepts in SSC CGL. This difference exists because compound interest includes interest on previously earned interest, while simple interest does not. Core Concept:
Simple Interest is calculated only on the principal amount throughout the investment period. Compound Interest is calculated on the principal plus accumulated interest from previous periods. The difference between CI and SI represents the 'extra earning' due to compounding effect.
Formula BlockMemorise — at least one formula appears in every paper
For 2 years: CI - SI = P × R² / (100)²
For 3 years: CI - SI = P × R² × (300 + R) / (100)³
Where P = Principal, R = Rate per annum, T = Time
Exam PatternsWhat examiners ask — read before attempting PYQs
SSC CGL typically asks three types of questions
(1) Direct calculation of difference given P, R, T (2) Finding principal when difference and rate are given (3) Finding rate when principal and difference are given. Most questions involve 2-3 years timeframe as longer periods make calculations complex
Powerful Shortcut for 2 Years
Difference = (SI for 1 year)² / Principal
This works because: If SI for 1 year = PRT/100, then difference = (PRT/100)² / P = PR²T²/(100²P) = PR²/100² (for T=2)
Worked ExampleSolve this step-by-step before moving on
1
Step 1
Calculate SI for 2 years
SI = (P × R × T) / 100 = (8000 × 15 × 2) / 100 = Rs. 2400
2
Step 2
Calculate CI for 2 years
Amount = P(1 + R/100)ᵀ = 8000(1 + 15/100)² = 8000 × (1.15)² = 8000 × 1.3225 = Rs. 10,580
CI = Amount - Principal = 10,580 - 8000 = Rs. 2580
3
Step 3
Find difference
CI - SI = 2580 - 2400 = Rs. 180
Alternative (Using Formula):
CI - SI = P × R² / (100)² = 8000 × (15)² / (100)² = 8000 × 225 / 10000 = Rs. 180
ShortcutsUse these to save 30–60 seconds per question
for 3 Years:
For 3 years, the difference equals: 3 × (2-year difference) + (2-year difference × R/100)
Exam TrapsCommon mistakes students make — avoid these
Students often confuse the formula for different time periods. Remember: the 2-year formula is simplest and most tested. For 3 years, don't memorize the complex formula - use the relationship with 2-year difference instead.
Key Points to Remember
Difference exists only when time period is more than 1 year
For 2 years: CI - SI = P × R² / (100)²
For 3 years: CI - SI = P × R² × (300 + R) / (100)³
Difference represents interest earned on interest portions
2-year difference problems are most common in SSC CGL
If SI for 1 year is known, 2-year difference = (SI)² / Principal
Compound Interest is always greater than Simple Interest for same P, R, T
The difference increases exponentially with higher rates and longer periods
Exam-Specific Tips
For 2 years at 10% rate, difference is always 1% of principal
For 2 years at 20% rate, difference is always 4% of principal
For Rs. 100 at 15% for 2 years, difference is exactly Rs. 2.25
The ratio CI:SI for 2 years at 10% is always 21:20
For 3 years, minimum additional factor in formula is 300 (when R=0)
Difference for 2 years = P×R²/10000 (direct calculation)
For equal principal and rate, 3-year difference is roughly 3 times 2-year difference
At 25% rate for 2 years, difference equals 6.25% of principal
Practice MCQs
Difference SI vs CI — Practice Questions
3graded MCQs · easy to hard · full solution & trap analysis
A sum of money is invested at 10% per annum. The difference between the compound interest and simple interest for 2 years is ₹40. Find the principal.
Practice 2medium
A sum of money is invested at 10% per annum. The difference between the compound interest and simple interest earned over 2 years is ₹40. What is the principal amount?
Practice 3hard
A sum of money is invested at 12% per annum. The difference between the compound interest for 3 years and the simple interest for 3 years is ₹1,123.20. Find the principal amount.
60-Second Revision — Difference SI vs CI
Formula: 2 years difference = P × R² / 10000
Remember: Difference exists only when T > 1 year
Trick: 2-year difference = (Annual SI)² / Principal
Pattern: Most SSC questions use 2-3 year timeframes
Trap: Don't use 3-year formula for 2-year problems
Quick check: At 10% for 2 years, difference = 1% of principal
Method: Calculate both CI and SI separately when formulas confuse