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Interest and Interest Rate. Interest ($) = amount owed now – original amount A) $1000 placed in bank account one year ago is now worth $1025. Interest earned is $25. $10,000 borrowed last year from Sharky’s Easy Money, and you now owe $12,000. Interest owed is $2000.

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interest and interest rate
Interest and Interest Rate

Interest ($) = amount owed now – original amount

A) $1000 placed in bank account one year ago is now worth $1025. Interest earned is $25.

  • $10,000 borrowed last year from Sharky’s Easy Money, and you now owe $12,000. Interest owed is $2000.

Interest paid over a specific time period is called an interest rate.

Interest rate (%) = interest accrued per time x 100%

original amount

What is the interest rate in example A and B?

rate of return
Rate of Return

Rate of Return (ROR) – interest earned over a specific time period, expressed as a percentage of the original amount.

Rate of return (%) = interest accrued per time x 100%

original amount

Borrower’s perspective – interest rate paid

Investor’s perspective – rate of return (ROR) or return on investment (ROI).

engineering economy
Engineering Economy

Other factors that act as interest:

  • Inflation
  • Appreciation
  • Depreciation
equivalence
Equivalence

Economic equivalence – different sums of money at different times can be equal in economic value because of the “time value of money” and interest rates.

Example: (assuming 5% interest rate)

$1000 today is equivalent to $1050 a year from now.

$1000(1.05) = $1050

$1000 a year from now is equivalent to $X today.

$X(1.05) = $1000

$X = $952.38

What is the interest rate if an investment of $500 is equivalent to $545 a year from now?

simple vs compound interest
Simple –vs- Compound Interest

Simple Interest – interest is calculated using the principal only.

interest = (principal)(number of periods)(interest rate)

Example: you invest $500 in an insurance policy that pays 8% simple interest. How much is the policy worth in 3 years?

PrincipalInterest

Year 0) $500

Year 1) $500 $40

Year 2) $500 $40

Year 3) $500 $40

$120

simple vs compound interest6
Simple –vs- Compound Interest

Compound Interest – interest is calculated using both the principal and interest earned.

interest = (principal + all accrued interest)(interest rate)

Example: you invest $500 in an insurance policy that pays 8% compound interest. How much is the policy worth in 3 years?

PrincipalInterestTotal

Year 0) $500

Year 1) $500 $40.00 $540 $500(1.08)1

Year 2) $540 $43.20 $583.2 $500(1.08)2

Year 3) $583.2 $46.65 $629.85 $500(1.08)3

$129.85

power of compound interest
Power of Compound Interest

Historical Perspective – In 1626, Manhattan Island was purchased from an Indian tribe for $24. If that tribe had invested the $24 in an investment paying 8% annually, what would it be worth today?

$24(1.08)(2011-1626) = $24(1.08)(385) = $24(7.38x10E12)

or $177 trillion

However, if the tribe invested in an investment that paid simple interest:

$24 + $24(.08)(385) = $763.20

symbols and terminology
Symbols and Terminology

$P – value or amount of money at time 0, also referred to as present worth (PW), present value (PV), net present value (NPV), or discounted cash flow (DCF).

$F – value or amount of money at some future time, also referred to as future worth (FW) or future value (FV).

$A – a series of consecutive, equal, end-of-period amount of money, also referred to as annual worth (AW) or equivalent uniform annual worth (EUAW). Does not have to be annual payouts, could be monthly.

n – number of periods; years, months, days

i% – interest rate or rate of return per time period

t – time, stated in periods

symbols and terminology9
Symbols and Terminology

Example: At the beginning of each year, my wife and I invest $2K in a college savings plan for our little boy. We did this beginning his first year and plan to do so until his 18th birthday. We hope to obtain a rate of return of 6% (our current rate is about –20%).

A = $2000

i = 6% or .06 annually

n = 18

F = ? the value of the investment when he begins college

minimum attractive rate of return marr
Minimum Attractive Rate of Return (MARR)

The Minimum Attractive Rate of Return is a minimum level set by a Corporation when deciding on whether to pursue or not to pursue projects.

Expected ROR

for a new proposal

MARR Is MARR constant from

year to year?

ROR on “safe”

Investment (e.g

money market)

cash flow diagrams
Cash Flow Diagrams

Cash flow diagrams are a way to graphically represent the inflows and outflows of cash over time.

Estimates of cash flows typically follow the end-of-period convention (cash flows are assumed to occur simultaneously at the end of an interest period). When several receipts and disbursements occur within an interest period, the new cash flow is depicted.

Net cash flow = receipts – disbursements

= cash inflows – cash outflows

cash flow diagrams12
Cash Flow Diagrams

Example:

  • $70K Purchase price with 20% down ($14K)
  • Monthly expenses (utilities, maintenance, insurance, etc..) - $200
  • Monthly P&I - $345
  • Rental income - $750

$50K

$50K

$750

$205

$200

$200

$545

$545

$14K

$14K

rule of 72 estimating doubling time and interest rate
Rule of 72: Estimating Doubling Time and Interest Rate

The rule of 72 helps estimate how long is will take for an investment, using compound interest, to double in value.

To estimate how many years,n (periods) to double your money:

estimated n = 72/i

To estimate the interest rate required to double your money in n periods:

estimated i = 72/n

rule of 72 estimating doubling time and interest rate14
Rule of 72: Estimating Doubling Time and Interest Rate

Example:

If you invested $1000 today in a CD paying 5% annually, how long will it be until the CD is worth $2000?

n = 72/5 = 14.4 years

If you wanted the $1000 to double in 10 years, what interest rate must you earn?

i = 72/10 = 7.2%