Simple Interest

Overview

Simple Interest (SI) is one of the most straightforward yet frequently tested topics in JTET Paper I Mathematics. It forms the foundation of commercial mathematics and helps students understand how money grows over time when borrowed or invested. At the primary level, students encounter SI problems that connect classroom learning with real-life situations—savings accounts, loans, and everyday financial transactions.

For JTET, you must master the basic formula, understand the relationship between Principal, Rate, and Time, and solve problems quickly using shortcuts. Questions typically involve direct application of the formula, finding any one of the four variables when three are given, or comparing two SI scenarios. This topic also builds the base for Profit-Loss and Percentage calculations, making it essential for overall exam success.

The key to scoring well is not just memorizing the formula but understanding what each term represents and how changes in one variable affect the final interest amount.

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Key Concepts

• **Principal (P)**: The original sum of money borrowed or invested before any interest is added. This is your starting amount.

• **Rate of Interest (R)**: The percentage charged or earned per year (per annum). Always expressed as "% per annum" unless stated otherwise.

• **Time (T)**: The duration for which money is borrowed or invested. Must be in years for the standard formula—convert months to years by dividing by 12.

• **Simple Interest (SI)**: The extra money paid or earned, calculated only on the original principal. Unlike compound interest, SI does not add interest to previous interest.

• **Amount (A)**: The total money after adding interest to the principal. Amount = Principal + Simple Interest.

• **SI is directly proportional to P, R, and T**: Doubling any one of these doubles the SI. This relationship is the basis for many shortcut problems.

• **Time conversion**: 6 months = 0.5 years, 3 months = 0.25 years, 18 months = 1.5 years. Always convert before applying the formula.

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Formulas / Key Facts

**Main Formula:** SI = (P × R × T) / 100

**Derived Formulas:**

• Principal: P = (SI × 100) / (R × T)
• Rate: R = (SI × 100) / (P × T)
• Time: T = (SI × 100) / (P × R)

**Amount Formula:** A = P + SI = P + (P × R × T) / 100 = P(1 + RT/100)

**Quick Facts:**

• If SI equals Principal, then R × T = 100