Rs.2700 becomes Rs.3600 at a certain rate of interest compounded annually in 5 years. Then find the amount at the end of 15th year.

The amount formula for compound interest is A = P(1 + r/100)ⁿ. Given that Rs. 2700 becomes Rs. 3600 in 5 years: 3600/2700 = 4/3 = (1 + r/100)⁵. After 15 years (three 5-year periods), the amount will be 2700 × (4/3)³ = 2700 × 64/27 = 6400. This matches option A. The key insight is that 15 years = 3 × 5 years, so we cube the 5-year growth factor.