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2. KRUGMAN’S
Economics
for AP®
S E C O N D E D I T I O N

3. Section 5
Module 24

4. What You Will Learn in this Module
Explain why a dollar today is worth more than a dollar a year from now
Use the concept of present value to make decisions about costs and benefits that come in the future
What You Will Learn in this Module
Section 5 | Module 24

5. The Concept of Present Value
When someone borrows money for a year, the interest rate is the price, calculated as a percentage of the amount borrowed, charged by the lender.
The interest rate can be used to compare the value of a dollar realized today with the value of a dollar realized later, because it correctly measures the cost of delaying a dollar of benefit (and the benefit of delaying a dollar of cost).
The present value of $1 realized one year from now is equal to $1/(1 + r): the amount of money you must lend out today in order to have $1 in one year. It is the value to you today of $1 realized one year from now.
Section 5 | Module 24

6. Present Value
Let’s call $X the amount of money you need to lend today, at an interest rate of r in order to have $1 in two years.
So if you lend $X today, you will receive $X(1 + r) in one year.
And if you re-lend that sum for yet another year, you will receive $X × (1 + r) × (1 + r) = $X × (1 + r)2 at the end of the second year.
At the end of two years, $X will be worth $X × (1 + r)2;
If r = 0.10, then this becomes $X × (1.10)2 = $X × (1.21).
Section 5 | Module 24

7. Present Value What is $1 realized two years in the future worth today?
In order for the amount lent today, $X, to be worth $1 two years from now, it must satisfy this formula:
$X × (1 + r)2 = $1
If r = 0.10, $X = $1/(1 + r)2 = $1/1.21 = $0.83
The present value formula is equal to $1/(1 + r)N
Section 5 | Module 24

8. Present Value Another example:
The process of finding present values is called discounting and the interest rate is called the discount rate.
EXAMPLE:
$100 received one year from now is $90.91(present value)=$100/( )
$100 received two years from now is $82.64=$100/(1+.10)2
Section 5 | Module 24

9. Using Present Value
The net present value of a project is the present value of current and future benefits minus the present value of current and future costs.
Section 5 | Module 24

10. F Y I How Big Is That Jackpot, Anyway?
On March 6, 2007, Mega Millions set the record for the largest jackpot ever in North America, for a payout of $390 million.
The $390 million was available only in the form of an “annuity” consisting of an annual payment for the next 26 years. If you wanted the cash up front, the jackpot was only $233 million and change.
If the winner took the annuity, the lottery would have invested the money in U.S. government bonds and using the prevailing interest rates at the time, the present value of a $390 million spread over 26 years was just about $233 million.
Section 5 | Module 24

11. Summary
To evaluate a project in which costs or benefits are realized in the future, you must first transform them into their present values using the interest rate, r.
The present value of $1 realized one year from now is $1/(1 + r), the amount of money you must lend out today to have $1 one year from now. Once this transformation is done, you should choose the project with the highest net present value.
Section 5 | Module 24