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engg economy

Engineering Economy 10 Dr. Emad Elbeltagi

CHAPTER 2

TIME VALUE OF MONEY

2.1 Concepts of Engineering Economics Analysis

Engineering Economy: is a collection of mathematical techniques which simplify

economic comparisons.

Time Value of Money: means that money has different value tomorrow than it has today.

Interest: is a measure of the increase between the original sum borrowed or invested and

the final amount owed or accrued. It is the growth in value with time.

Interest = Total amount accumulated Original investment (2.1)

On the other hand, if you had borrowed money from the bank at some time in the past:

Interest = Present amount owed Original loan (2.2)

In either case, there is an increase in the amount of money that was originally invested or

borrowed, and the increase over the original amount is the interest.

The original investment or loan: is referred to as Principal.

2.1.1 Interest Calculations

When interest is expressed as a percentage of the original amount per unit time, the result

is what is called an interest rate. The most common period or unit of time is one year.

% interest rate = (Interest accrued per unit of time / Original amount) 100% (2.3)

Example 2.1: Suppose you invested LE100,000 on May 1, and withdraw a total of

EL106,000 exactly one year later. Compute the interest and the interest

rate.

Solution: Interest = LE106,000 - LE100,000 = LE6,000

Engineering Economy 11 Dr. Emad Elbeltagi

Percent interest rate = (LE6,000 per year / LE100,000) 100% = 6% per

year

2.1.2 Equivalence

When we are indifferent as to whether we have a quantity of money now or the assurance

of some other sum of money in the future, or series of future sums of money, we say that

the present sum of money is equivalent to the future sum of series of future sums.

Equivalence is an essential factor in engineering economic analysis.

Different sums of money at different times can be equal in economic value. For example,

if the interest rate is 6% per year, a LE100 today (present time) would be equivalent to

LE106 one year from today. Also, LE100 today is equivalent to LE94.34 one year ago.

Therefore, LE94.34 last year, LE100 now, and LE106 one year from now are equivalent

when the interest rate is 6% per year. In this example, all calculations are made based on

6% interest rate, so changing the interest rate would change the payments. As such, the

equivalence is dependent on the interest rate.

2.1.3 Rate of Return

The rate of return is used when determining the profitability of a proposed investment or

past investment.

Rate of return (RR) = (Total amount of money received Original investment) /

Original investment 100%

= (Profit / Original investment) 100% (2.4)

Interest rate is used when borrowing capital or when a fixed rate has been established.

2.2 Time Value of Money

The value of money is dependent on the time at which it is received. A sum of money on

hand today is worth more than the same sum of money to be received in the future

because the money on hand today can be invested to earn interest to gain more than the

same money in the future. Thus, studying the present value of money (or the discounted

Engineering Economy 12 Dr. Emad Elbeltagi

value) that will be received in the future is very important.

The money consequences of any alternative occur over a long period of time, a year or

more. When money consequences occur in a short period of time, we simply add up the

various sums of money and obtain a net result. But we cannot treat money this same way

when the time span is longer. Which would you prefer, LE100 cash today or receiving

LE100 a year from now? A little thought should convince you that it is desirable to

receive the LE100 now, rather than a year from now. This is because, you might consider

leaving the LE100 in a bank if you know it would be worth LE109 one year from now.

To facilitate the computations of interest formulas, the following notations will be used:

i = interest rate per interest period (usually calculated annually), it is stated as a

decimal (e.g., 9% interest is 0.09).

n = number of interest periods.

P = A present sum of money.

F = A future sum of money.

A = A series of periodic, equal amount of money. This is always paid or received

at end of period.

2.3 Single Payment

The Future Value of a given present value of money represents the amount, at some time

in the future, that an investment made today will grow to if it is invested at a specific

interest rate. For example, if you were to deposit LE100 today in a bank account to earn

an interest rate of 10% compounded annually, this investment will grow to LE110 in one

year. The investment earned LE10. At the end of year two, the current balance LE110

will be invested and this investment will grow to LE121 [110 x (1 + 0.1)].

2.3.1 Simple Interest

Simple interest is calculated using the principal only (i.e. the original investment or

original loan), ignoring any interest that has been accrued in preceding interest periods.

Engineering Economy 13 Dr. Emad Elbeltagi

Simple interest is seldom used in todays economics. Thus, if you were to loan a present

sum of money P to someone at a simple annual interest rate i for a period of n years, the

amount of interest you would receive from the loan would be:

Total Interest = Principal (P) Number of periods (n) Interest rate (i)

= P n i (2.5)

At the end of n years the amount of money due F would equal the amount of the loan P

plus the total interest earned.

F = P + P n i

= P (1 + i n) (2.6)

Example 2.2: If you borrow LE1,000 for three years at 6% per year simple interest,

how much money will you owe at the end of three years?

Solution: Simple Interest = Principal (P) Number of periods (n) Interest rate (i)

= LE1,000 3 0.06 = LE180

Amount due after three years = LE1,000 + LE180 = LE1,180

2.3.2 Compound Interest

Compound interest is calculated using the principal plus the total amount of interest

accumulated in previous periods. Thus, compound interest means interest on top of

interest.

Example 2.3: If you borrow LE1,000 at 6% per year compound interest, compute the

total amount owed after three year period?

Solution: Interest for Year 1 = LE1,000 0.06 = LE60

Total amount due after year 1 = LE1,000 + LE60 = LE1,060

Interest for year 2 = LE1,060 0.06 = LE63.60

Total amount due after year 2 = LE1,060 + LE63.60 = LE1,123.60

Interest for year 3 = LE1,123.60 0.06 = LE67.42

Total amount due after year 3 = LE1,123.60 + LE67.42 = LE1,191.02

Thus, with compound interest, the original LE1,000 would accumulate an

extra LE1,191.02 - LE1,180 = LE11.02 compared to simple interest in the

three year period.

Engineering Economy 14 Dr. Emad Elbeltagi

Therefore, if an amount of money P is invested at some time t = 0, the total amount of

money (F) that would be accumulated after one year would be:

F1 = P + Pi

F1 = P(1 + i)

Where F1 = the total amount accumulated after one year

At the end of the second year, the total amount of money accumulated (F2) would be

equal to the total amount that had accumulated after year 1 plus interest from the end of

year 1 to the end of year 2.

F2 = F1 + F1 i

F2 = P(1 + i) + P(1 + i)i

F2 = P(1 + i) (i +i)

F2 = P(1 + i)2

Similarly, the total amount of money accumulated at the end of year 3 (F3) would be

equal to the total amount that had accumulated after year 2 plus interest from the end of

year 2 to the end of year 3.

F3 = F2 + F2 i

F3 = P(1 + i)2 + P(1 + i)

2 i

F3 = P(1+ i)2 (1 + i)

F3 = P(1 + i)3

From the preceding values, it is evident that by mathematical induction that the formula

for calculating the total amount of money (F) after (n) number of years, using compound

interest (i) would be:

F = P(1 + i)n (2.7)

This is the single-payment compound amount formula and the values between the

parenthesis (1 + i)n is called the single-payment compound amount factor. This

equation could be written in the functional notation as:

Engineering Economy 15 Dr. Emad Elbeltagi

F = P (F/P, i, n) (2.8)

Equation 2.9 is read as: find a future sum F given a present sum P at an

interest rate i per period and n interest periods.

A present sum P can be determined, easily, knowing a future sum F by rewriting

Equation 2.7 as follows:

P = F / (1 + i)n = F (1 + i)

-n (2.9)

This is the single-payment present worth formula and the values between the

parenthesis [1 /