How long will ₹9000 take to earn ₹2160 as simple interest at 8% per annum?

Class 8Comparing QuantitiesComparing Quantities

Step-by-Step Solution

Interest for one year = 8% of 9000 = 720, so the time =

\frac{2160}{720} = 3

years. Trap: 24 years divides 2160 by

\frac{9000}{100}

and drops the rate altogether; 0.24 divides the interest by the principal alone; 8 years just copies the rate.

Answer: B) 3 years

After year 1, interest is added to principal — so year 2's interest is calculated on a LARGER amount, not the original.

**

A = P \times (1 + R /100)^{T}

**, where A is the final amount after

T

years.
**CI =

A - P

** (the total interest earned).

Example: ₹2000 at 5% for 3 years, compounded annually:
- Year 1:

2000 \times 1.05 = 2100 -

Year 2:

2100 \times 1.05 = 2205 -

Year 3:

2205 \times 1.05 = 2315.25

Direct formula:

A = 2000 \times (1.05)^{3} = 2315.25

. CI = 2315.25 − 2000 = 315.25 (vs SI of 300 — compound gives more).

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