Simple Interest is one of the easiest and most frequently asked topics in quantitative aptitude. Whether you’re preparing for TCS NQT, Infosys, Accenture, Wipro, SSC, Banking, CAT, Railways, or other competitive exams, understanding Simple Interest can help you score quick marks.

Most Simple Interest questions are formula-based and require only basic arithmetic. With a strong grasp of the formula and a few shortcut techniques, you can solve many questions in under a minute.

In this guide, you’ll learn:

• What Simple Interest is
• Important formulas
• Formula summary table
• Solved examples
• Shortcut tricks
• Common mistakes
• 30 practice questions
• FAQs

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What is Simple Interest?

Simple Interest (SI) is the interest calculated only on the original principal amount. Unlike Compound Interest, the interest does not get added back to the principal after each period.

Example

If you invest ₹10,000 at 8% per annum for 3 years, the interest is calculated only on ₹10,000 every year.

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Simple Interest at a Glance

Term	Meaning
Principal (P)	Original amount borrowed or invested
Rate (R)	Annual interest rate (%)
Time (T)	Duration in years
Interest (SI)	Money earned or paid
Amount (A)	Principal + Interest

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Simple Interest Formula

Formula

SI = (P × R × T) / 100

Where:

• P = Principal
• R = Rate of Interest (%)
• T = Time (Years)

Amount Formula

Amount = Principal + Simple Interest

or

A = P + SI

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Formula Summary

Find	Formula
Simple Interest	SI = (P × R × T) / 100
Amount	A = P + SI
Principal	P = (SI × 100) / (R × T)
Rate	R = (SI × 100) / (P × T)
Time	T = (SI × 100) / (P × R)

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When to Use the Simple Interest Formula

Simple Interest is commonly used in:

• School aptitude questions
• Placement aptitude tests
• Banking exams
• Personal loans
• Short-term loans
• Some fixed deposits
• Financial literacy calculations

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Solved Examples

Example 1 – Find Simple Interest

Question

A sum of ₹8,000 is invested at 10% per annum for 3 years.

Solution

SI = (8000 × 10 × 3) / 100
   = ₹2,400

Answer: ₹2,400

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Example 2 – Find Amount

Question

• Principal = ₹12,000
• Rate = 5%
• Time = 4 years

Solution

SI = (12000 × 5 × 4) / 100
   = ₹2,400

Amount = 12,000 + 2,400
       = ₹14,400

Answer: ₹14,400

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Example 3 – Find Principal

Question

• Interest = ₹900
• Rate = 6%
• Time = 3 years

Solution

P = (900 × 100) / (6 × 3)

P = ₹5,000

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Example 4 – Find Rate

Question

• Principal = ₹10,000
• Interest = ₹2,500
• Time = 5 years

Solution

R = (2500 × 100) / (10000 × 5)

R = 5%

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Example 5 – Find Time

Question

• Principal = ₹8,000
• Interest = ₹1,600
• Rate = 10%

Solution

T = (1600 × 100) / (8000 × 10)

T = 2 years

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Example 6 – Time in Months

Question

• Principal = ₹12,000
• Rate = 12%
• Time = 18 months

Convert months into years.

18 months = 18 / 12 = 1.5 years

SI = (12000 × 12 × 1.5) / 100

SI = ₹2,160

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Example 7 – Time in Days

Question

• Principal = ₹5,000
• Rate = 10%
• Time = 180 days

Convert days into years.

180 / 365 ≈ 0.493 years

SI ≈ ₹247

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Example 8 – Word Problem

Ravi borrowed ₹15,000 at 8% simple interest for 2 years.

Interest = (15000 × 8 × 2) / 100
         = ₹2,400

Total Amount = ₹17,400

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Example 9 – Placement Style Question

A sum becomes ₹11,500 after 3 years at 10% simple interest.

Find the principal.

Let Principal = P

Amount = P + 30% of P

Amount = 1.3P

1.3P = 11,500

P = ₹8,846.15

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Example 10 – Mixed Aptitude Question

A sum doubles in 20 years under simple interest.

Find the annual rate.

Interest = Principal

R = 100 / 20

R = 5%

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Simple Interest vs Compound Interest

Simple Interest	Compound Interest
Calculated only on principal	Calculated on principal plus accumulated interest
Linear growth	Exponential growth
Easier calculations	More complex calculations
Mostly used in aptitude basics	Used in advanced finance

Coming Soon: Compound Interest Aptitude Guide

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Pocket Aptitude Speed Tips

Tip 1

Cancel common factors before multiplying.

Example:

(5000 × 12 × 5) / 100

5000 ÷ 100 = 50

50 × 12 × 5

This saves time during exams.

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Tip 2

Memorize common percentages.

• 5%
• 10%
• 20%
• 25%
• 50%

These frequently appear in aptitude questions.

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Tip 3

Always convert months into years before applying the formula.

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Tip 4

Write the formula first.

This reduces calculation mistakes.

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Common Exam Traps

Many students lose easy marks because of small mistakes.

Avoid these:

• Forgetting to convert months into years
• Confusing Amount with Interest
• Ignoring the phrase per annum
• Dividing incorrectly by 100
• Using the Compound Interest formula instead of Simple Interest
• Writing the wrong unit for time

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Quick Revision Table

Concept	Formula
SI	(P × R × T) / 100
Amount	P + SI
Principal	(SI × 100) / (R × T)
Rate	(SI × 100) / (P × T)
Time	(SI × 100) / (P × R)

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Practice Questions

Beginner (1–10)

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Intermediate (11–20)

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Placement Level (21–30)

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Answers

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Frequently Asked Questions

1. What is Simple Interest?

Simple Interest is interest calculated only on the original principal amount.

2. What is the formula for Simple Interest?

SI = (P × R × T) ÷ 100

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3. What is the difference between Simple Interest and Compound Interest?

Simple Interest is calculated only on the principal, while Compound Interest is calculated on the principal plus previously earned interest.

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4. Can time be given in months?

Yes. Convert months into years by dividing by 12.

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5. Can time be given in days?

Yes. Convert days into years, usually by dividing by 365 (or 366 in a leap year if specified).

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6. Is the interest rate always annual?

In aptitude questions, the rate is usually given per annum (per year) unless stated otherwise.

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7. Which exams ask Simple Interest questions?

TCS NQT, Infosys, Accenture, Wipro, SSC, Banking, Railways, CAT, MAT, CMAT, and many state-level competitive exams.

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8. Can the interest rate change every year?

The standard Simple Interest formula assumes a constant rate. If the rate changes each year, calculate the interest separately for each period and add the results.

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9. How can I solve Simple Interest questions faster?

Memorize the formulas, simplify calculations by cancelling common factors before multiplying, and always convert the time into years before applying the formula.

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10. Is Amount the same as Interest?

No.

• Interest is the extra money earned or paid.
• Amount is the total of the principal and the interest.

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Summary

Simple Interest is one of the highest-scoring topics in quantitative aptitude because the concepts are straightforward and formula-based. By understanding the core formulas, practicing different question types, avoiding common mistakes, and using shortcut techniques, you can solve most Simple Interest problems quickly and accurately.

If you’ve already learned Percentage, Profit & Loss, and Discount, Simple Interest becomes even easier because it builds directly on percentage calculations. Continue practicing regularly, and then move on to Compound Interest to strengthen your aptitude skills further.