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Find the simple interest on ₹15000 at 8% per annum for 9 months.

Class 8Comparing QuantitiesComparing Quantities
  • A₹1200
  • B₹10800
  • C₹100
  • D₹900

Step-by-Step Solution

Time must be in years: 9 months = year. SI = . Trap: ₹1200 charges a full year, ₹10800 puts years into the formula, and ₹100 is the interest for a single month.
Answer: D) ₹900

Key idea: Compound interest — interest on interest

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

****, where A is the final amount after years.
**CI = ** (the total interest earned).

Example: ₹2000 at 5% for 3 years, compounded annually:
- Year 1: Year 2: Year 3:

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

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