If Rs.P amounts to Rs.4P in 12 years at compound interest, the rate per annum is approximately

1. 4P = P(1 + r)^12, so (1 + r)^12 = 4. 2. Take 12th root: 1 + r = 4^(1/12) = 2^(1/6). 3. 2^(1/6) is approximately 1.1225. 4. r = 0.1225 = 12.25% per annum. _Source: ICAI BoS Foundation Paper 3, Ch 4 'Math of Finance', p.9_