Learning / Teaching Financial Math via Quantitative Interactive Workbook Courseware QIWCourseware by Dan DuPort

1. Using FNemli
Determining the best decision
when confronted with two
investment alternatives.

2. Using FNemli
• The following (4 slides) illustrates how beginning
finance students can learn about using value in
decision making.
How would you tell someone about
the equal periodic amount you can withdraw
from a saving account and be left with 0 after N
withdrawals?
• Most beginning mathematics of finance texts simply
state or derive formulas and then drill students on
solving for the various variables given the values of the
others in the formula. But the real heart of the
mathematics of finance is the use of the concepts to
provide insight into making value judgments.

3. Given that I can receive an amount in the future, FV, then
the value of that amount today being worth PV = FV /
(1+i)N is a fundamental principle of Finance.
It stems from the idea the PV can be invested in a fund
with compound interest i, for N periods to achieve the
amount FV. I emphasis the word "can" and the idea of
"in principle". The value of the amount in the future is
independent of what is actually done with the money
that you get along the way. So even though the actual
amounts, P, may not be invested, the value of the
receiving of them is still given by the future value
formula for FV.
A regular annuity's future value with payment P, interest i
per period, for N periods is:
FV = P [ (1+i)0 + (1+i)1 + (1+i)2 + …. + (1+i)N-1 ]
= P [ (1+i)N – 1 ] / i

4. When a loan of amount A is taken out by an individual
or a commercial business, an interest rate, i, and a
term, N, are what determines the payment amount, P.
As we know the Payment P must be such that
A = the present value of the future value, FV, of the
annuity created by P. FV = P [ (1+i)N – 1 ] / i , as
announced on the previous page. Its present value,
PV, is
FV / (1+i)N ={P [ (1+i)N – 1 ] / i }/ (1+i)N
= P [ 1 - (1+i)-N ] / i
Why is that?
The lender of the loan has two choices. The lender
can, instead of making the loan, invest the loan
amount at the rate i compounded for N periods. But if
the loan is made, the lender can reinvest each
payment as received into an annuity of interest rate i,
for N periods, to get the same future value.

5. For example, to make this really clear, if you were to
loan a friend 1000 dollars, and the friend was
trustworthy in that the payments would be made on
time, then instead leaving the 1000 in a bank account
and obtaining compound interest at i% per period for N
periods, you could give your friend the 1000 dollars
and have your friend pay you N equal payments of an
amount P, deposit each in an annuity when received,
and after N periods, you would have 1000(1+i)N
because
FV = P [ (1+i)N – 1 ] / i = ( P [ 1 - (1+i)-N ] / i ) (1+i)N
= PV (1+i)N = 1000(1+i)N
You have lost nothing by making this loan to your
friend. This illustrates the fundamental rule of
equalization in finance: If I have A (dollars) and if I can
get à one way, and get the same à a different way,
then there is no choice to make between the two if the
risk involved in getting à is the same in both cases.

6. It is just this principle that allows you to make finance
decisions and also to solve financial problems.
How can I use this equalization rule to determine the
amount of money, A, I need today to be able to draw
an amount P from it every period for N periods if the
assumed interest rate is i per period? We argue as
before:
If I have A today, then in N periods I can obtain
A (1+i)N by placing A in the bank. On the other hand,
if I get P every period, for N periods, I can build an
annuity of N payments with future value
FV = P [ (1+i)N – 1 ] / i .
After the N withdrawals, the two must be equal.
So A (1+i)N = P [ (1+i)N – 1 ] / i.
And therefore
A = P [ 1 - (1+i)-N ] / i is simply the present value of
the annuity that I would get by depositing the P each
period into an account, even though I might not do
that.

7. Using FNemli
Download a copy of FNemli from
QIWCourseware.com
Use it for free, no strings attached, for
learning the basics of Mathematics of
Finance. 28 pages of interactive, visual
learning that’s easy on the mind – from TVM
to MIRR.
Requires Excel 2010 for the PC
Excel 2011 for the MAC
or higher