1. VOCABULARY WORD DESCRIPTION Principal Interest Interest Rate
WORD BANK
Compound Interest
Interest
Interest Rate
Principal
Rule of 72
Simple Interest
Term
WORD
DESCRIPTION
Principal
1. The original amount of money deposited
Interest
2. Money an institution pays you for use of your funds
3. Expressed as a percentage, what an account will earn if funds are kept on deposit for an agreed-upon term
Interest Rate
4. Interest paid only on the principal amount deposited into the account
Simple Interest
5. The method of computing interest where the interest rate is applied to the principal and any earned interest; often referred to as “interest on interest”
Compound Interest
Term
6. Length of time money left on deposit in account
7. A calculation that estimates growth of funds over time with compound interest
Rule of 72 1

2. When you deposit money into a bank, the bank pays you interest.
When you borrow money from a bank, you pay interest to the bank.
Simple interest is money paid only on the principal.
Rate of interest is the percent charged or earned.
I = P  r  t
Time that the money is borrowed or invested (in years).
Principal is the amount of money borrowed or invested.

3. Additional Example 1: Finding Interest and Total Payment on a Loan
To buy a car, Jessica borrowed $15,000 for 3 years at an annual simple interest rate of 9%. How much interest will she pay if she pays the entire loan off at the end of the third year? What is the total amount that she will repay?
First, find the interest she will pay.
I = P  r  t Use the formula.
I = 15,000  0.09  3 Substitute. Use 0.09 for 9%.
I = Solve for I.

4. Additional Example 1 Continued
Jessica will pay $4050 in interest.
You can find the total amount A to be repaid on a loan by adding the principal P to the interest I.
P + I = A principal + interest = total amount
15, = A Substitute.
19,050 = A Solve for A.
Jessica will repay a total of $19,050 on her loan.

5. Check It Out! Example 1
To buy a laptop computer, Elaine borrowed $2,000 for 3 years at an annual simple interest rate of 5%. How much interest will she pay if she pays the entire loan off at the end of the third year? What is the total amount that she will repay?
First, find the interest she will pay.
I = P  r  t Use the formula.
I = 2,000  0.05  3 Substitute. Use 0.05 for 5%.
I = Solve for I.

6. Check It Out! Example 1 Continued
Elaine will pay $300 in interest.
You can find the total amount A to be repaid on a loan by adding the principal P to the interest I.
P + I = A principal + interest = total amount
= A Substitute.
2300 = A Solve for A.
Elaine will repay a total of $2300 on her loan.

7. Additional Example 2: Determining the Amount of Investment Time
Nancy invested $6000 in a bond at a yearly rate of 3%. She earned $450 in interest. How long was the money invested?
I = P  r  t Use the formula.
450 = 6000  0.03  t Substitute values into the equation.
450 = 180t
2.5 = t Solve for t.
The money was invested for 2.5 years, or 2 years and 6 months.

8. Check It Out! Example 2
TJ invested $4000 in a bond at a yearly rate of 2%. He earned $200 in interest. How long was the money invested?
I = P  r  t Use the formula.
200 = 4000  0.02  t Substitute values into the equation.
200 = 80t
2.5 = t Solve for t.
The money was invested for 2.5 years, or 2 years and 6 months.

9. Additional Example 3: Computing Total Savings
John’s parents deposited $1000 into a savings account as a college fund when he was born. How much will John have in this account after 18 years at a yearly simple interest rate of 3.25%?
I = P  r  t Use the formula.
I = 1000   Substitute. Use for 3.25%.
I = Solve for I.
The interest is $585. Now you can find the total.

10. Additional Example 3 Continued
P + I = A Use the formula.
= A Substitute.
1585 = A Solve for A.
John will have $1585 in the account after 18 years.

11. Check It Out! Example 3
Bertha deposited $1000 into a retirement account when she was 18. How much will Bertha have in this account after 50 years at a yearly simple interest rate of 7.5%?
I = P  r  t Use the formula.
I = 1000   Substitute. Use for 7.5%.
I = Solve for I.
The interest is $3750. Now you can find the total.

12. Check It Out! Example 3 Continued
P + I = A Use the formula.
= A Substitute.
4750 = A Solve for A.
Bertha will have $4750 in the account after 50 years.

13. Additional Example 4: Finding the Rate of Interest
Mr. Johnson borrowed $8000 for 4 years to make home improvements. If he repaid a total of $10,320, at what interest rate did he borrow the money?
P + I = A Use the formula.
I = 10,320 Substitute.
I = 10,320 – 8000 = 2320 Subtract from both sides.
He paid $2320 in interest. Use the amount of interest to find the interest rate.

14. Additional Example 4 Continued
I = P  r  t Use the formula.
2320 = 8000  r  Substitute.
2320 = 32,000  r Simplify.
2320
32,000
= r Divide both sides by ,000.
= r
Mr. Johnson borrowed the money at an annual rate of 7.25%, or 7 %. 1 4

15. Check It Out! Example 4
Mr. Mogi borrowed $9000 for 10 years to make home improvements. If he repaid a total of $20,000 at what interest rate did he borrow the money?
P + I = A Use the formula.
I = 20,000 Substitute.
I = 20,000 – 9000 = 11,000 Subtract from both sides.
He paid $11,000 in interest. Use the amount of interest to find the interest rate.

16. Check It Out! Example 4 Continued
I = P  r  t Use the formula.
11,000 = 9000  r  10 Substitute.
11,000 = 90,000  r Simplify.
11,000
90,000
= r Divide both sides by 90,000.
0.12 = r
Mr. Mogi borrowed the money at an annual rate of about 12.2%.

17. Compound interest is interest paid not only on the principal, but also on the interest that has already been earned. The formula for compound interest is below.
A = P(1 + )
r n nt
A is the final dollar value, P is the principal, r is the rate of interest, t is the number of years, and n is the number of compounding periods per year.

18. The table shows some common compounding periods and how many times per year interest is paid for them.
Compounding Periods
Times per year (n)
Annually 1
Semi-annually 2
Quarterly 4
Monthly 12

19. Additional Example 5: Applying Compound Interest
David invested $1800 in a savings account that pays 4.5% interest compounded semi-annually. Find the value of the investment in 12 years.
A = P(1 + )
r n nt
Use the compound interest formula.
= 1800( )
t 2
2(12)
Substitute.
= 1800( )24 Simplify.
= 1800(1.0225)24 Add inside the parentheses.

20. Additional Example 5 Continued
≈ 1800( ) Find (1.0225)24 and round.
≈ 3, Multiply and round to the nearest cent.
After 12 years, the investment will be worth about $3,

21. Use the compound interest formula.
Check It Out! Example 5
Kia invested $3700 in a savings account that pays 2.5% interest compounded quarterly. Find the value of the investment in 10 years.
A = P(1 + )
r n nt
Use the compound interest formula.
= 3700( )
t 4
4(10)
Substitute.
= 3700( )40 Simplify.
= 3700( )40 Add inside the parentheses.

22. Check It Out! Example 5 Continued
≈ 3700( ) Find ( )40 and round.
≈ 4, Multiply and round to the nearest cent.
After 10 years, the investment will be worth about $4,

23. Lesson Quiz: Part I
1. A bank is offering 2.5% simple interest on a savings account. If you deposit $5000, how much interest will you earn in one year?
2. Joshua borrowed $1000 from his friend and paid him back $1050 in six months. What simple annual interest did Joshua pay his friend?

24. Lesson Quiz: Part I
1. A bank is offering 2.5% simple interest on a savings account. If you deposit $5000, how much interest will you earn in one year?
2. Joshua borrowed $1000 from his friend and paid him back $1050 in six months. What simple annual interest did Joshua pay his friend?
$125
10%

25. Lesson Quiz: Part II
3. The Hemmings borrowed $3000 for home improvements. They repaid the loan and $600 in simple interest four years later. What simple annual interest rate did they pay?
4. Theresa invested $800 in a savings account that pays 4% interest compounded quarterly. Find the value of the investment after 6 years.

26. Lesson Quiz: Part II
3. The Hemmings borrowed $3000 for home improvements. They repaid the loan and $600 in simple interest four years later. What simple annual interest rate did they pay?
4. Theresa invested $800 in a savings account that pays 4% interest compounded quarterly. Find the value of the investment after 6 years. 5% $

27. Lesson 4: Back to School
Watch it Grow: Rule of 72
With the rule of 72, you can estimate the growth of funds over time with compound interest.
You calculate the length of time (in years) for a principal deposit to double.
You divide 72 by the rate of return.
EXAMPLE:
Say you deposit $5,000 today at an 8% interest rate.
Apply the rule: 72 ÷ 8 = 9
The principal will double every 9 years. 27

28. Rule of 72 Calculations: Problem #1
Lesson 4: Back to School
Rule of 72 Calculations: Problem #1
If you deposit $50,000, how many years will it take for it to grow to $100,000?
At 4% annual interest
72 ÷ 4 = 18 years
At 6% annual interest
72 ÷ 6 = 12 years
At 9% annual interest
72 ÷ 9 = 8 years
At 12% annual interest
72 ÷ 12 = 6 years 28

29. Rule of 72 Calculations: Problem #2
Lesson 4: Back to School
Rule of 72 Calculations: Problem #2
What interest rate do you need to grow $50,000 to $100,000?
In 2 years?
72 ÷ 2 = 36%
In 5 years?
72 ÷ 5 = 14.4%
In 10 years?
72 ÷ 10 = 7.2%
In 20 years?
72 ÷ 20 = 3.6% 29