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Mathematics of Finance

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Mathematics of Finance

Simple Interest and the Future Value


Interest Rates are generally quoted in percentage form and for use in calculation, must be
converted to the equivalent decimal value by dividing the percentage by 100.
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For example, i = 8 4 % = 8.25% =8.25/100 =.0825.
Unless otherwise stated, a quoted rate is a rate per year.
Formula for calculating Simple Interest.

I = pin,
where, I = interest in taka/dollar
p = principal, the sum of money on which interest is being earned.
i = Rate of interest per period ( assumed to be one year).
n = Number of years or fraction of one year.
Example:
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Find the interest on $600 at 7 2 % for 10 months.
Solution: I = pin, Here p = $600, i= 7 12 % =15/2% = 7.5/100 = .075
n =10 months = 10/12
I =pin = (600)(.075)(10/12) = $37.50

Exercise: Compute the interest on $480 at 6- 1/4 % for 9 months. Ans: $22.50
The simple interest formula involves four variables. Given any three of the four variables, the
fourth can be found by solving the formula.

Example: Find the interest rate, if $1000 earns $45 interest in 6 months.

I = pin Here, I=$45 p=$1000 n = 6 months = 6/12 =.5


45=1000(i)(.5) 45=500i i=45/500=.09(Decimal Rate) To obtain the percent rate we
multiply by 100, i=.09x100=9%
The Future Value
The Future value is the sum of principal plus interest.
Formula for the future value:
F = p + pin = p(1 + in)
Example: Find the future value if $20,000 is invested at 6 percent for 3 months.

F = p(1+in) Here p = 20,000 i=6% = 6/100 = .06 n =3 months = 3/12 = ¼


F =20000[1+(.06)(1/4)] =20000(1+.015) =$20,300

Exercise: Find the future value of $5000 at 10 percent for 9 months. Ans: $5375.00
Future Value Formula involves four variables. Given any three, fourth can be found.
Example: Jan received $50 for a diamond at a Jewelry shop and a month later paid $53.50 to
get the diamond back. Find the percent interest rate.
Solution: F =p(1+in)
Here p =$50 F = $53.50, n =1/12year
53.50 = 50[1+(i)(1/12)]
Dividing both sides by 50
53.50/50 =(1+i/12)
12(53.50/50) = 12+I
12.84 = 12+I
i=.84
So, i=.84x100= 84%
Exercise: Frank has placed $500 in an employees’ savings account that pays 8 percent
simple interest. How long will it be,in months, until the investment amounts to $530?
Present Value
Present value is analogous to the principal
F=p(1+in) Divide both sides by (+in)
F/(1+in) = p
P=F/(1+in) In practice, accountants and financiers use the present value concept very
often.

Example: Find the present value of $530 receivable 9 months from now if the interest rate is 8
percent.

P=F/(1+in) = 530/[1+.08(9/12)]=530/1.06 =$500


Exercise: How much will Fran have to invest now in the employees’ 8 percent savings account
in order to have $600 a year from now?
Compund Interest and the Future value
Compound interest includes interest on the interest.

Future Value,
F = p(1+i)n It is the future value of $p for n periods at an interest rate of i percent.

Example: Find the future value of $1000 at 7 percent compounded yearly for 10 years.

Solution:

F = p(1+i)n here,
p=$1000 i=7% = 7/100 = .07 n=10years.

F =1000(1+.07)10 =1000(1.07)10 = $1967.15


EXERCISE: If $500 is invested at 6 percent compounded annually,
what will be the future value 30 years later? Ans:$2871.74
The Conversion Period

Quoted interest rates are rates per year if not accompanied by a qualifying statement such as 1.5
percent per month.
In the absence of a qualifier, the quoted annual rate is called the nominal rate.
Nominal Rate = Rate per year.
Although the nominal rate is the rate per year, it is common practice to compound interest more
frequently than once a year.
Many banks compound interest on savings accounts on a daily basis (365 times a year).
In other transactions, interest is compounded monthly, quarterly, or semiannually.
No. of conversions per year
Conversion No. of conversions per year
Daily 365
Monthly 12
Quarterly 4
Semiannually 2
Example
Find the future value of $500 at 8 percent compounded quarterly for 10 years.

Solution: F = P(1+i)n

Here, no. of conversions per year = 4


So n = (No. of years)(No. of conversions per year) = (10)(4) = 40
The quoted 8% is a nominal rate or rate per year.

i = Nominal rate per year/No. of conversions per year = 8%/4 = 2% =2/100 =.02 per period.
F=500(1+.02)40 =500(1.02)40 =$1104.02
Exercise: If $500 is invested at 6 percent compounded semiannually , what will be the amount
5 years?
Example: Compute the future value of $5,000at 9 percent compounded monthly for 10 years.

Solution: F = P(1+i)n
Here P=$5000 n=(10)(12) = 120 i = .09/12 = .0075

F = 5000(1+.0075)120 =$12256.79

Exercise: A bank pays 7.25 percent compounded daily on 90-day notice accounts. If $500is
deposited in such an account, what will be the amount in 90 days? Ans. :$509.02
Finding the time and the interest rates
The compound interest formula involves four variables. If we know one, the other three can be found.

Example: At 8 percent compounded annually, how many years will it take for $2000 to grow to $3000?

Solution: F=p(1+i)n n?

3000=2000(1+.08)n 2000(1.08)n = 3000


(1.08)n =3000/2000 =1.5
Taking natural logarithms on both sides, ln(1.08) n = ln(1.5)
n ln(1.08) = ln(1.5)
n = ln1.5/ln1.08 = 5.268 years.
.268 years .268*12 =3.216 months. .216 months = .216*30 =6.48 days =6 days.
It will take 5 years 3 months and 6 days.
Exercise: Find how many years it will take at 9 percent compounded annually for $1000 to grow to $2000.
Ans: About 8.043 years.
Example: At what interest rate compounded annually will a sum of money double in 10 years?
Solution: Here doubling means $100 grows to $200, $25 grows to $50, $1 grows to $2 and so
on and the time required is the same in any doubling.
We shall use p= $1 and F = $2
Hence F =p(1+i)n 2 =1 (1+i)10 1(1+i)10 =2 (1+i)10 =2
Taking natural logarithms on both sides, 10ln(1+i) =ln2 ln(1+i) = ln2/10=.069314718

(1+i) =Antiln(.069314718) =1.071773462 So, i=1.071773462-1 = .071773462

i= .071773462*100 =7.177%
Exercise: Find the rate of interest that, compounded annually, will result in tripling a sum of
money in 10 years?
Ans. 11.612 percent.

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