In how many years will ₹7200 earn ₹1296 as simple interest at 4.5% per annum?

Class 7Comparing QuantitiesComparing Quantities

A3 years
B4.5 years
C5 years
D4 years

Step-by-Step Solution

Rearrange

SI = \frac{P \times R \times T}{100}

. Substituting,

1296 = \frac{7200 \times 4.5 \times T}{100} = 324 T

, so

T = \frac{1296}{324} = 4

years. Check:

\frac{7200 \times 4.5 \times 4}{100} = \frac{129600}{100} = 1296

. The 4.5-year option is the trap of copying the rate into the time slot —

R

and

T

are different quantities even when one of them looks convenient.

Answer: D) 4 years

Key idea: Simple interest

When you lend (or borrow) money, the borrower pays the lender extra for the use of the money — that extra is **interest**. **Simple interest** is calculated only on the original principal, not on past interest.

**

SI = \frac{P \times R \times T}{100}

**

where

P

= principal (₹),

R

= rate per annum (% per year),

T

= time in years.

Example: ₹2000 at 5% for 3 years: SI =

\frac{2000 \times 5 \times 3}{100} = 300

. Total amount

A = 2000 + 300

= ₹2300.

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