Simple Interest

Overview

Simple Interest (SI) is one of the most straightforward yet frequently tested topics in OTET Paper I Mathematics. It forms the foundation of commercial mathematics and helps students understand how money grows over time when borrowed or invested. This concept directly connects classroom learning to real-life situations like bank deposits, loans, and installment purchases.

For OTET, you must master the basic formula, understand what each variable represents, and solve word problems quickly. Questions typically involve finding any one of the four quantities (SI, Principal, Rate, or Time) when the other three are given. The topic also appears in combination with problems involving Amount, which adds Principal and Interest together.

Since this is tested at the primary level (Classes I-V context), the numerical calculations remain simple, but conceptual clarity is essential for both solving problems and teaching young learners effectively.

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Key Concepts

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

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

• **Time (T):** The duration for which money is borrowed or invested. Usually given in years; if given in months, convert to years by dividing by 12.

• **Simple Interest (SI):** The extra money paid or earned on the principal. In simple interest, this is calculated only on the original principal, not on accumulated interest.

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

• **"Per Annum" means "per year":** When rate is 5% per annum, it means 5% of principal is charged as interest each year.

• **Interest remains constant each year:** Unlike compound interest, simple interest gives the same interest amount every year because it is always calculated on the original principal.

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Formulas / Key Facts

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

Where:

• SI = Simple Interest
• P = Principal (in rupees)
• R = Rate of interest (percent per annum)
• T = Time (in years)

**Derived Formulas:**

• P = (SI × 100) / (R × T) — To find Principal

• R = (SI × 100) / (P × T) — To find Rate