Study Material — 5 PYQs (2018–2022) · Concept Notes · Shortcuts
CDS Simple Interest is a frequently tested subtopic — 5 previous year questions from 2018–2022 papers are included below with concept notes, key rules and shortcut tricks.
5 questions from actual CDS papers · all shown free · click option to reveal solution
Exam Q 12018Previous Year Pattern
A sum of ₹5,000 is invested at a simple interest rate of 8% per annum. What will be the simple interest earned after 3 years?
Exam Q 22022Previous Year Pattern
A sum of ₹5,000 is invested at a simple interest rate of 8% per annum. What will be the interest earned after 3 years?
Exam Q 32022Previous Year Pattern
At what rate per annum will ₹2,400 amount to ₹3,120 in 3 years under simple interest?
Exam Q 42018Previous Year Pattern
A sum of ₹8,400 is lent at a simple interest rate of 12.5% per annum. In how many years will the interest amount to ₹3,150?
Exam Q 52020Previous Year Pattern
Rajesh borrowed some money at 8% per annum simple interest. After 3 years, he paid back ₹4,160 as interest. He then borrowed the same principal amount at 6% per annum simple interest for a certain number of years. If the interest earned in the second case is ₹3,120, find the difference (in years) between the two time periods.
Concept Notes
Simple Interest— Rules & Concept
Core ConceptRead this first — the foundation of the topic
Simple Interest is the extra money paid on borrowed money or earned on invested money. It is calculated only on the original amount (called Principal) for a specific time period at a fixed rate. Core Concept: Simple Interest remains constant every year. If you borrow Rs. 1000 at 10% simple interest, you pay Rs. 100 every year as interest. The principal amount never changes in calculations.
Formula BlockMemorise — at least one formula appears in every paper
Block:
Simple Interest (SI) = (P × R × T) / 100
Amount = Principal + Simple Interest
Principal (P) = (SI × 100) / (R × T)
Rate (R) = (SI × 100) / (P × T)
Time (T) = (SI × 100) / (P × R)
Exam PatternsWhat examiners ask — read before attempting PYQs
SSC CGL consistently asks 2-3 questions on Simple Interest. Common question types include finding SI when P, R, T are given, calculating time or rate when other values are known, and comparing simple vs compound interest scenarios.
Master Shortcut #1 - Quick SI Calculation:
For easy percentages, use direct multiplication:
- 10% of any amount = Amount/10
- 5% of any amount = Amount/20
- 20% of any amount = Amount/5
Worked ExampleSolve this step-by-step before moving on
1
Step 1
Identify P = 8000, R = 12%, T = 3 years
2
Step 2
Apply formula SI = (P × R × T) / 100
3
Step 3
SI = (8000 × 12 × 3) / 100
4
Step 4
SI = 288000 / 100 = Rs. 2880
5
Step 5
Amount = 8000 + 2880 = Rs. 10,880
Shortcut #2 - Time-Rate Relationship:
If rate doubles, time becomes half for same SI.
If time doubles, rate becomes half for same SI.
This helps eliminate wrong options quickly.
Worked Example 2:
Question: At what rate will Rs. 5000 amount to Rs. 6500 in 4 years at simple interest?
1
Step 1
Amount = 6500, Principal = 5000
2
Step 2
SI = Amount - Principal = 6500 - 5000 = Rs. 1500
3
Step 3
Using R = (SI × 100) / (P × T)
4
Step 4
R = (1500 × 100) / (5000 × 4)
5
Step 5
R = 150000 / 20000 = 7.5%
Shortcut #3 - Percentage Method:
When principal becomes 'n' times in 't' years:
Rate = [(n-1) × 100] / t
Example: If money doubles (n=2) in 10 years, Rate = (2-1) × 100/10 = 10%
Most Common Trap: Students often confuse the time unit. If rate is per annum but time is given in months, convert months to years by dividing by 12. Always match the time unit with the rate unit. This single mistake costs many marks in SSC CGL.
Another frequent error is adding interest multiple times. Remember, in simple interest, you add interest only once to get the final amount, unlike compound interest where interest compounds.
Key Points to Remember
Simple Interest formula: SI = (P × R × T) / 100 where P=Principal, R=Rate, T=Time
Amount = Principal + Simple Interest (add only once, not yearly)
SI remains constant every year unlike compound interest which grows
Quick calculation: 10% SI = Principal/10, 20% SI = Principal/5
If money becomes n times in t years, Rate = [(n-1) × 100] / t
Time and rate are inversely proportional for same SI amount
Always convert time units to match rate units (months to years or vice versa)
Principal can be found using: P = (SI × 100) / (R × T)
Rate doubles means time halves for same SI amount earned
Common trap: Never compound the interest in simple interest problems
Exam-Specific Tips
Standard SI formula uses division by 100, never by 1000 or other numbers
When rate is given per annum, time must be in years for direct calculation
If principal doubles, the rate-time product always equals 100
SI for 2 years at 10% rate equals 20% of principal amount
Principal formula derivation: P = (SI × 100) / (R × T)
Rate formula: R = (SI × 100) / (P × T) gives percentage value
Time formula: T = (SI × 100) / (P × R) gives time in rate's unit
Amount formula: A = P + SI = P[1 + (R × T)/100]
Practice MCQs
Simple Interest — Practice Questions
53graded MCQs · easy to hard · full solution & trap analysis · showing 20 of 53
A principal of ₹8,000 earns ₹1,600 as simple interest in 4 years. What is the rate of interest per annum?
Practice 2easy
Rajesh borrowed ₹12,000 at 5% simple interest per annum. If he paid ₹2,400 as interest, for how many years did he borrow the money?
Practice 3easy
At what rate of simple interest per annum will ₹2,400 amount to ₹3,120 in 5 years?
Practice 4easy
The simple interest on ₹8,000 for 2.5 years at a certain rate is ₹2,000. What is the rate of interest?
Practice 5easy
A sum of money doubles itself in 5 years at simple interest. What is the rate of interest per annum?
Practice 6easy
Rahul borrowed ₹12,000 at 5% simple interest per annum. If he paid ₹15,000 as total amount after some years, how many years did he borrow the money for?
Practice 7easy
In how many years will ₹6,000 become ₹7,800 at a simple interest rate of 6% per annum?
Practice 8easy
A principal amount becomes ₹8,400 after 4 years at 10% simple interest per annum. What was the original principal?
Practice 9easy
At what rate of simple interest per annum will ₹8,000 amount to ₹10,400 in 4 years?
Practice 10easy
A sum of ₹5,000 is invested at a simple interest rate of 8% per annum. What will be the interest earned after 3 years?
Practice 11easy
A sum of money becomes ₹9,600 in 2 years and ₹12,000 in 4 years at simple interest. What is the principal?
Practice 12easy
At what rate of simple interest per annum will ₹8,000 amount to ₹9,600 in 2 years?
Practice 13easy
A sum of ₹5,000 is invested at a simple interest rate of 8% per annum. What will be the total amount after 3 years?
Practice 14easy
In how many years will ₹5,000 amount to ₹6,800 at 12% per annum simple interest?
Practice 15easy
A principal amount becomes ₹6,500 in 2 years and ₹7,500 in 4 years at simple interest. What is the principal amount?
Practice 16easy
Rajesh invested ₹8,000 at a simple interest rate of 12% per annum. How much interest will he earn in 3 years?
Practice 17easy
A principal amount becomes ₹10,500 in 2 years and ₹12,000 in 4 years at simple interest. What is the principal amount?
Practice 18medium
A sum of money becomes ₹9,240 in 4 years at 8% per annum simple interest. What was the original principal?
Practice 19medium
A sum of ₹8,000 is invested at 12% per annum simple interest. How much interest will be earned in 3 years and 6 months?
Practice 20medium
A sum of money becomes ₹8,400 in 2 years and ₹9,600 in 4 years at simple interest. What is the principal amount?
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