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**(c) 2002 Contemporary Engineering Economics**

Chapter 4 Time Is Money Interest: The Cost of Money Economic Equivalence Development of Interest Formulas Unconventional Equivalence Calculations (c) 2002 Contemporary Engineering Economics

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**Key Concepts of Last Lecture**

Interest Principal? Interest rate? Interest period?- number of interest periods. Plan for receipts or disbursements Future amount of money Simple vs Compound (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Time Value of Money Money has a time value because it can earn more money over time (earning power). Time value of money is measured in terms of interest rate. Interest is the cost of money—a cost to the borrower and an earning to the lender (c) 2002 Contemporary Engineering Economics

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**Elements of Transactions involve Interest**

Initial amount of money in transactions involving debt or investments is called the principal. The interest rate measures the cost or price of money and is expressed as a percentage per period of time. A period of time, called the interest period, determines how frequently interest is calculated. A specified length of time marks the duration of the transactions and thereby establishes a certain number of interest periods. A plan for receipts or disbursements that yields a particular cash flow pattern over a specified length of time. A future amount of money results from the cumulative effects of the interest rate over a number of interest periods. (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

An = A discrete payment or receipt occurring at the end of some interest period. i = The interest rate per interest period. N = The total number of interest periods. P = a sum of money at a time chosen for purposes of analysis as time zero, sometimes referred to as the present value or present worth. F = A future sum of money at the end of the analysis period. This sum may be specified as Fn. A = An end of period payment or receipt in a uniform series that continues for N periods. This is a special situation where A1 = A2 = …= AN. V n = An equivalent sum of money at the end of a specified period n that considers the effect of time value of money. Note that V0 = P and VN = F. (c) 2002 Contemporary Engineering Economics

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**Example of an Interest Transaction Repayment Plans (Table 4.1)**

End of Year Receipts Payments Plan 1 Plan 2 Year 0 $20,000.00 $200.00 Year 1 5,141.85 Year 2 Year 3 Year 4 Year 5 30,772.48 P = $20,000, A = $5,141.85, F = $30,772.48 (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Cash Flow Diagram Represent time by a horizontal line marked off with the number of interest periods specified. Cash flow diagrams give a convenient summary of all the important elements of a problem. (c) 2002 Contemporary Engineering Economics

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**Methods of Calculating Interest**

Simple interest: the practice of charging an interest rate only to an initial sum (principal amount). Compound interest: the practice of charging an interest rate to an initial sum and to any previously accumulated interest that has not been withdrawn. (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Simple Interest P = Principal amount i = Interest rate N = Number of interest periods Example: P = $1,000 i = 8% N = 3 years I=(iP)N End of Year Beginning Balance Interest earned Ending Balance $1,000 1 $80 $1,080 2 $1,160 3 $1,240 (c) 2002 Contemporary Engineering Economics

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**Compound Interest (Example 4.1)**

P = Principal amount i = Interest rate N = Number of interest periods Example: P = $1,000 i = 8% N = 3 years F=P(1+i)^N End of Year Beginning Balance Interest earned Ending Balance $1,000 1 $80 $1,080 2 $86.40 $1,166.40 3 $93.31 $1,259.71 (c) 2002 Contemporary Engineering Economics

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**Comparing Simple to Compound Interest**

(c) 2002 Contemporary Engineering Economics

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**Simple Vs Compound Interest**

(c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Economic Equivalence What do we mean by “economic equivalence?” Why do we need to establish an economic equivalence? How do we establish an economic equivalence? (c) 2002 Contemporary Engineering Economics

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**Economic Equivalence (EE)**

Economic equivalence exists between cash flows that have the same economic effect and could therefore be traded for one another. EE refers to the fact that a cash flow-whether a single payment or a series of payments-can be converted to an equivalent cash flow at any point in time. Even though the amounts and timing of the cash flows may differ, the appropriate interest rate makes them equal. (c) 2002 Contemporary Engineering Economics

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**Typical Repayment Plans for a Bank Loan of $20,000**

Repayments Plan 1 Plan 2 Plan 3 Year 1 $5,141.85 $1,800.00 Year 2 5,141.85 1,800.00 Year 3 Year 4 Year 5 $30,772.48 21,800.00 Total of payments $25,709.25 $29,000.00 Total interest paid $5,709.25 $10,772.48 $9,000.00 (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

If you deposit P dollars today for N periods at i, you will have F dollars at the end of period N. F dollars at the end of period N is equal to a single sum P dollars now, if your earning power is measured in terms of interest rate i. N F P (c) 2002 Contemporary Engineering Economics

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**Equivalence Between Two Cash Flows**

Step 1: Determine the base period, say, year 5. Step 2: Identify the interest rate to use. Step 3: Calculate equivalence value. $3,000 $2,042 5 (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Example 4.3 Equivalence Various dollar amounts that will be economically equivalent to $3,000 in 5 years, given an interest rate of 8% (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Example 4.4 (Prnpl:1) Equivalent Cash Flows are Equivalent at Any Common Point In Time (c) 2002 Contemporary Engineering Economics

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**The Five Types of Cash Flows**

Single cash flow Equal (uniform) payment series Linear gradient series Geometric gradient series Irregular payment series (c) 2002 Contemporary Engineering Economics

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**Single Cash Flow Formula (Example 4.7)**

Single payment -compound amount factor (growth factor) Given: Find: F N P (c) 2002 Contemporary Engineering Economics

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**Single Cash Flow Formula (Example 4.8)**

Single payment present worth factor (discount factor) Given: Find: F N P (c) 2002 Contemporary Engineering Economics

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**Uneven Payment Series (example 4.11)**

(c) 2002 Contemporary Engineering Economics

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**Example 4.12 Calculating the Actual Worth of a Long-Term Contract**

(c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Equal Payment Series A N N F P (c) 2002 Contemporary Engineering Economics

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**Equal Payment Series Compound Amount Factor**

N A Example 4.13: Given: A = $3,000, N = 10 years, and i = 7% Find: F Solution: F = $3,000(F/A,7%,10) = $41,449.20 (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Sinking Fund Factor is an interest-bearing account into which a fixed sum is deposited each interest period; it is commonly established for the purpose of replacing fixed assets. F N A Example 4.15: Given: F = $5,000, N = 5 years, and i = 7% Find: A Solution: A = $5,000(A/F,7%,5) = $869.50 (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Sinking fund Sinking fund: 1) A fund accumulated by periodic deposits and reserved exclusively for a specific purpose, such as retirement of a debt or replacement of a property. 2) A fund created by making periodic deposits (usually equal) at compound interest in order to accumulate a given sum at a given future time for some specific purpose. (c) 2002 Contemporary Engineering Economics

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**Capital Recovery Factor**

N A Example 4.16: Given: P = $250,000, N = 6 years, and i = 8% Find: A Solution: A = $250,000(A/P,8%,6) = $54,075 (c) 2002 Contemporary Engineering Economics

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**Capital Recovery Factor (Annuity Factor)**

Annuity: 1) An amount of money payable to a recipient at regular intervals for a prescribed period of time out of a fund reserved for that purpose. 2) A series of equal payments occurring at equal periods of time. 3) Amount paid annually, including reimbursement of borrowed capital and payment of interest. Annuity factor: The function of interest rate and time that determines the amount of periodic annuity that may be paid out of a given fund. (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Equal Payment Series Present Worth Factor Present worth: The equivalent value at the present, based on time value of money. P N A Example 4.18: Given: A = $32,639, N = 9 years, and i = 8% Find: P Solution: P = $32,639(P/A,8%,9) = $203,893 (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Linear Gradient Series Engineers frequently meet situations involving periodic payments that increase or decrease by a constant amount (G) from period to period. refer to book, Equation 4.17 P (c) 2002 Contemporary Engineering Economics

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**Gradient Series as a Composite Series**

(c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Example 4.20 $2,000 $1,750 $1,500 $1,250 $1,000 How much do you have to deposit now in a savings account that earns a 12% annual interest, if you want to withdraw the annual series as shown in the figure? P =? (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Method 1: $2,000 $1,750 $1,500 $1,250 $1,000 $1,000(P/F, 12%, 1) = $892.86 $1,250(P/F, 12%, 2) = $996.49 $1,500(P/F, 12%, 3) = $1,067.67 $1,750(P/F, 12%, 4) = $1,112.16 $2,000(P/F, 12%, 5) = $1,134.85 $5,204.03 P =? (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Method 2: (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Geometric Gradient Series Many engineering economic problems, particularly those relating to construction costs, involve cash flows that increase over time, not by a constant amount, but rather by a constant percentage (geometric), called compound growth. (c) 2002 Contemporary Engineering Economics

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**Geometric Gradient: Find P, Given A1,g,i,N**

N = 5 years A1 = $54,440 Find: P Using equation 4.26 in the text, we obtain P=$222,283 (c) 2002 Contemporary Engineering Economics

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**(c) 2002 Contemporary Engineering Economics**

Summary Money has a time value because it can earn more money over time. Economic equivalence exists between individual cash flows and/or patterns of cash flows that have the same value. Even though the amounts and timing of the cash flows may differ, the appropriate interest rate makes them equal. The purpose of developing various interest formulas was to facilitate the economic equivalence computation. (c) 2002 Contemporary Engineering Economics

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