(7) Lesson 2.8

1. Course 2, Lesson 2-8
Find the sale price to the nearest cent.
1. $120 jacket; 30% discount
2. $10,500 car; 10% discount
3. $35 sweater; 18% discount; 3% tax
4. An employee at a restaurant receives a 60% discount on meals. If the
total before the discount is $32.50, how much does the employee pay?
5. A shirt is advertised “Buy 1, Get 1 50% off.” If the original price of every
shirt is $12.98 and you take advantage of the special offer, what is the
price you would pay per shirt?
6. A hotel room costs $155 per night. What is the total cost with a 30%
discount and 5.5% sales tax?

2. Course 2, Lesson 2-8
ANSWERS
1. $84
2. $9,450
3. $29.56
4. $13
5. $9.74 per shirt
6. $114.47

3. HOW can percent help you understand
situations involving money?
Ratios and Proportional Relationships
Course 2, Lesson 2-8

4. Ratios and Proportional Relationships
Course 2, Lesson 2-8 Common Core State Standards © Copyright 2010. National Governors Association Center for Best Practices and Council
of Chief State School Officers. All rights reserved.
• 7.RP.3
Use proportional relationships to solve multistep ratio and percent problems.
• 7.EE.3
Solve multistep real-life and mathematical problems posed with positive and
negative rational numbers in any form (whole numbers, fractions, and
decimals), using tools strategically. Apply properties of operations to
calculate with numbers in any form; convert between forms as appropriate;
and assess the reasonableness of answers using mental computation and
estimation strategies.

5. Ratios and Proportional Relationships
Course 2, Lesson 2-8 Common Core State Standards © Copyright 2010. National Governors Association Center for Best Practices and Council
of Chief State School Officers. All rights reserved.
Mathematical Practices
1 Make sense of problems and persevere in solving them.
3 Construct viable arguments and critique the reasoning of others.
4 Model with mathematics.

6. How to use the simple interest formula to:
• Find interest earned on saving accounts
• Find interest owed on loans and credit
cards
Ratios and Proportional Relationships
Course 2, Lesson 2-8

7. Ratios and Proportional Relationships
Course 2, Lesson 2-8
• principal
• simple interest

8. Ratios and Proportional Relationships
Course 2, Lesson 2-8
Words Simple interest l is the product of the principal p, the
annual interest rate r, and the time t, expressed in
years.
Symbols l = prt

9. 1
Need Another Example?
2
3
4
Step-by-Step Example
1. Arnold puts $580 into a savings account. The account pays 3%
simple interest. How much interest will he earn in each amount of time?
5 years
Formula for simple interest.
Replace p with $580, r with 0.03, and t with 5.
Simplify.
/ = prt
/ = 580 0.03 5
/ = 87
So, Arnold will earn $87 in interest in 5 years.

10. Answer
Need Another Example?
Jose’s savings account pays 3.25% interest.
He has $1,200 in his account. How much
interest will he earn in 3 years?
$117

11. 1
Need Another Example?
2
3
4
5
Step-by-Step Example
2. Arnold puts $580 into a savings account. The account pays
3% simple interest. How much interest will he earn in each
amount of time?
6 months
6 months = or 0.5 year Write the time as years.
Formula for simple interest.
p = $580, r = 0.03, t = 0.5
Simplify.
/ = prt
/ = 580 0.03 0.5
/ = 8.7
So, Arnold will earn $8.70 in interest in 6 months.

12. Answer
Need Another Example?
Jose’s savings account pays 3.25% interest.
He has $1,200 in his account. How much
interest will he earn in 9 months?
$29.25

13. 1
Need Another Example?
2
3
4
Step-by-Step Example
Rondell’s parents borrow $6,300 from the bank for a new
car. The interest rate is 6% per year. How much simple
interest will they pay if they take 2 years to repay the loan?
3.
Formula for simple interest.
Replace p with $6,300, r with 0.06, and t with 2.
Simplify.
/ = prt
/ = 6,300 0.06 2
/ = 756
Rondell's parents will pay $756 in interest in 2 years.

14. Answer
Need Another Example?
Phoebe borrowed $2,600 from a bank to help
pay for her college tuition. The interest rate is
8% per year. How much simple interest will she
pay if it takes her 5 years to repay the loan?
$1,040

15. Need Another Example?
Step-by-Step Example
1
2
3
4
4. Derrick’s dad bought new tires for $900 using a credit card.
His card has an interest rate of 19%. If he has no other charges
on his card and does not make a payment, how much money
will he owe after one month?
Formula for simple interest.
Simplify.
/ = prt
/ = 900 0.19
/ = 14.25
The interest owed after one month is $14.25.
So, the total amount owed would be $900 + $14.25 or $914.25.
Replace p with $900, r with 0.19, and t with .

16. Answer
Need Another Example?
Mariano paid for a plane ticket that cost $365
using a credit card. His card has an interest rate
of 13.5%. If he has no other charges on his card
and does not pay off his balance by the end of
the month, how much money will he owe after
one month?
$369.11

17. 1
Need Another Example?
2
3
4
5
6
Step-by-Step Example
5. Luis is taking out a car loan for $5,000. He plans on paying off
the car loan in 2 years. At the end of 2 years, Luis will have paid
$300 in interest. What is the simple interest rate on the car loan?
Formula for simple interest.
Replace l with 300, p with 5,000, and t with 2.
Simplify.
/ = prt
300 = 5,000 r 2
300 = 10,000r
Divide each side by 10,000.
0.03 = r
The simple interest rate is 0.03 or 3%.

18. Answer
Need Another Example?
Callie borrowed $4,500 from a bank to pay part
of her college tuition. She will repay the loan by
paying $120 a month for the next four years.
What is the simple interest rate on the loan?
7%

19. How did what you learned
today help you answer the
HOW can percent help you understand
situations involving money?
Course 2, Lesson 2-8
Ratios and Proportional Relationships

20. How did what you learned
today help you answer the
HOW can percent help you understand
situations involving money?
Course 2, Lesson 2-8
Ratios and Proportional Relationships
Sample answers:
• Interest rates are written as percents.
• You can find the amount of simple interest earned on a
savings account.
• You can find the amount of interest charged on bank
loans and credit cards.

21. Would you prefer to deposit
$1,000 at a simple interest
rate of 3% for 5 years or
$1,000 at a simple interest
rate of 4% for 4 years?
Explain your reasoning.
Ratios and Proportional RelationshipsRatios and Proportional Relationships
Course 2, Lesson 2-8