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Compound growth of savings or investments. Interest: definition. A. a sum paid or charged for the use of money or for borrowing money B. such a sum expressed as a percentage of money borrowed to be paid over a given period, usually one year. Source: dictionary.com. Two types of interest.
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Interest: definition • A. a sum paid or charged for the use of money or for borrowing money • B. such a sum expressed as a percentage of money borrowed to be paid over a given period, usually one year. Source: dictionary.com
Two types of interest • Simple interest: Fixed percentage of original amount invested or deposited. • Compound interest: Fixed percentage of original amount plus accumulated interest.
Simple v. compound • Simple interest = linear growth • Compound interest = exponential growth
Definitions: • Balance: the current value of an account; equivalent to Y in exponential growth formula • Principle: initial amount invested or deposited; equivalent to X in exponential growth formula
Basic formula for compound growth Balance=Principle(1+r)y Where: y = number of time periods of growth r= interest rate or rate of return (percentage expressed as decimal)
Solving for principle Balance/(1+r)y = Principle
Solving for y log (Balance/Principle)/log(1+r) = y
Solving for r ((Balance/principle)^(1/y))-1 = r
Compounding more than yearly Balance=Principle (1+r/n)yn
Compounding quarterly Balance=Principle (1+r/4)4y
Compounding monthly Balance=Principle (1+r/12)12y
5% APR, compounded quarterly, for 7 years Balance=Principle (1+.05/4)28
5% APR, compounded monthly, for 7 years Balance=Principle (1+.05/12)84
Annual percentage yield [APY] • In formulas, r was annual percentage rate or APR • When interest gets compounded more often than once per year, actual interest earned in a year is greater than APR
Example: $10,000 invested for 10 years at 8% APR Annually: $21,589.25 Quarterly: $22,080.40 Monthly: $22,196.40
Computing APY • Compute the balance for two consecutive years at comparable point in time. • Calculate percentage change: (new balance-old balance)/old balance
Demonstration Chart to simulate growth in value of: • $10,000 investment • at 3% APR • compounded monthly Open Excel file