1. Percent Increase and Decrease
8-4
Percent Increase and Decrease
Warm Up
Problem of the Day
Lesson Presentation
Pre-Algebra

2. Percent Increase and Decrease
Pre-Algebra
8-4
Percent Increase and Decrease
Warm Up
1. 14,000 is 2 % of what number?
2. 39 is 13% of what number?
% of what number is 12?
4. 150% of what number is 189? 1 2
560,000
300 1 2 32
126

3. Problem of the Day
In a school survey, 45% of the students said orange juice was their favorite juice, 25% preferred apple, and 10% preferred grapefruit. The remaining 32 students preferred grape juice. How many students participated in the survey?
160 students

4. Learn to find percent increase and decrease.

5. Vocabulary
percent change
percent increase
percent decrease

6. Percents can be used to describe a change
Percents can be used to describe a change. Percent change is the ratio of the amount of change to the original amount.
amount of change
original amount
percent change =
Percent increase describes how much the original amount increases.
Percent decrease describes how much the original amount decreases.

7. Additional Example 1: Percent Increase and Decrease
Find the percent increase or decrease from 16 to 12.
This is percent decrease.
16 – 12 = 4 First find the amount of change.
Think: What percent is 4 of 16?
amount of decrease
original amount 4 16
Set up the ratio.

8. Additional Example 1 Continued4 16
= Find the decimal form.
= 25% Write as a percent.
From 16 to 12 is a 25% decrease.

9. This is percent increase.
Try This: Example 1
Find the percent increase or decrease from 15 to 20.
This is percent increase.
20 – 15 = 5 First find the amount of change.
Think: What percent is 5 of 20?
amount of increase
original amount 5 20
Set up the ratio.

10. Try This: Example 1 Continued5 20
= Find the decimal form.
= 25% Write as a percent.
From 15 to 20 is a 25% increase.

11. Additional Example 2: Life Science Application
A. When Jim was exercising, his heart rate went from 70 beats per minute to 98 beats per minute. What was the percent increase?
98 – 70 = 28 First find the amount of change.
Think: What percent is 28 of 70?
amount of increase
original amount 28 70
Set up the ratio.

12. Additional Example 2 Continued
= 0.4 Find the decimal form. 70 28
= 40% Write as a percent.
Jim’s heart rate increased by 40% when he exercised.

13. Additional Example 2B: Application
B. In 1999, a certain stock was worth $1.25 a share. In 2002, the same stock was worth $0.85 a share. What was the percent decrease?
1.25 – 0.85 = 0.40 First find the amount of change.
Think: What percent is 0.40 of 1.25?
amount of decrease
original amount
1.25
0.40
Set up the ratio.

14. Additional Example 2B Continued
1.25
0.40
= Find the decimal form.
= 32% Write as a percent.
The value of the stock decreased by 32%.

15. 75 – 50 = 25 First find the amount of change.
Try This: Example 2A
A. When Jeff was watching TV, the number of times his eyelids blinked went from 50 blinks per minute to 75 blinks per minute. What was the percent increase?
75 – 50 = 25 First find the amount of change.
Think: What percent is 25 of 50?
amount of increase
original amount 25 50
Set up the ratio.

16. Try This: Example 2A Continued
= 0.5 Find the decimal form. 50 25
= 50% Write as a percent.
The blinking of Jeff’s eyelids increased by 50% when he watched TV.

17. Try This: Example 2B
B. In 2000, a certain stock was worth $9.00 a share. In 2003, the same stock was worth $3.80 a share. What was the percent decrease?
9.00 – 3.80 = 5.20 First find the amount of change.
Think: What percent is 5.20 of 9.00?
amount of decrease
original amount
9.00
5.20
Set up the ratio.

18. Try This: Example 2B Continued
9.00
5.20
= Find the decimal form.
= 57.7% Write as a percent.
The value of the stock decreased by about 57.8%.

19. Additional Example 3A: Percent Increase and Decrease
A. Sarah bought a DVD player originally priced at $450 that was on sale for 20% off. What was the sale price?
$450  20% First find 20% of $450.
$450  0.20 = $90 20% = 0.20
The amount of decrease is $90.
Think: The reduced price is $90 less than $450.
$450 – $90 = $360 Subtract the amount of decrease.
The sale price of the DVD player was $360.

20. Additional Example 3B: Percent Increase and Decrease
B. Mr. Olsen has a computer business in which he sells everything at 40% above the wholesale price. If he purchased a printer for $85 wholesale, what will be the retail price?
$85  40% First find 40% of $85.
$85  0.40 = $34 40% = 0.40
The amount of increase is $34.
Think: The retail price is $34 more than $85.
$85 + $34 = $119 Add the amount of increase.
The retail price of this printer will be $119.

21. Try This: Example 3A
A. Lily bought a dog house originally priced at $750 that was on sale for 10% off. What was the sale price?
$750  10% First find 10% of $750.
$750  0.10 = $75 10% = 0.10
The amount of decrease is $75.
Think: The reduced price is $75 less than $750.
$750 – $75 = $675 Subtract the amount of decrease.
The sale price of the dog house was $675.

22. Try This: Example 3B
B. Barb has a grocery store in which she sells everything at 50% above the wholesale price. If she purchased a prime rib for $30 wholesale, what will be the retail price?
$30  50% First find 50% of $30.
$30  0.50 = $15 50% = 0.50
The amount of increase is $15.
Think: The retail price is $15 more than $30.
$30 + $15 = $45 Add the amount of increase.
The retail price of the prime rib will be $45.

23. Lesson Quiz
Find each percent increase or decrease to the nearest percent.
1. from 12 to 15
2. from 1625 to 1400
3. from 37 to 125
4. from 1.25 to 0.85
5. A computer game originally sold for $40 but is now on sale for 30% off. What is the sale price of the computer game?
25% increase
14% decrease
238% increase
32% decrease
$28