Compound interest

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Compound interest is paid on the original principal and accumulated part of interest.
Formula for calculating compound interest :
P = A(1 +r/n)^nt, where
P = the principal
A = the amount deposited
r = the rate (expressed as fraction, e.g. 6 per cent = 0.06)
n = number of times per year that interest is compounded
t = number of years invested
Frequently compounding of Interest. If the interest is compounded :
Annually = P (1 + r)
Quarterly = P (1 + r/4)^4
Monthly = P (1 + r/12)^12
Example :
The compound interest on Rs. 30,000 at 7% per annum is Rs. 4347. The period (in years) is:
Amount = Rs. (30000 + 4347) = Rs. 34347.
Let the time be n years. Then
30000(1+7/100)^n = 34347
(107/100)^n = 34347/30000
(107/100)^n = 11449/10000
(107/100)^n = (107/100)^2
n = 2 years.
The Rule of 72: Allows you to determine the number of years before your money doubles whether in debt or investment. Divide the number 72 by the percentage rate.