At a certain simple rate of interest, a given sum amounts to Rs 13920 in 3 years, and to Rs 18960 in 6 years and 6 months. If the same given sum had been invested for 2 years at the same rate as before but with interest compounded every 6 months, then the total interest earned, in rupees, would have been nearest to
A
Step-by-Step Solution
Key idea: this is a two-stage interest question, recognisable because it first gives two SI amounts to find the principal and rate, and then applies CI with half-yearly compounding.
Step 1: Use the SI data to find Principal () and Rate ().
- Amount after 3 years = 13920.
- Amount after 6.5 years = 18960.
Step 2: The difference in time is years. The difference in amount is purely due to SI on for 3.5 years.
Step 3: SI for 3.5 years = .
Step 4: SI for 1 year = .
Step 5: SI for 3 years = .
Step 6: Since Amount = + SI, .
Step 7: Find the annual rate : .
Step 8: Now, is invested for 2 years at 15% p.a., compounded half-yearly.
Step 9: For half-yearly compounding, the rate per period is , and the number of periods is .
Step 10: Calculate the final Amount .
Step 11: . Squaring again: .
Step 12: .
Step 13: Total interest earned = .
Nearest to 3221.
Answer: Option A.