8-4

8-4 Percent Increase and Decrease

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Warm UpProblem of the DayLesson Presentation

Warm Up

1. 14,000 is 2 % of what number?2. 39 is 13% of what number?3. 37 % of what number is 12?4. 150% of what number is 189?

560,00030032

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12

12612

Problem of the DayIn 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

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Learn to find percent increase and decrease.

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Vocabularypercent changepercent increasepercent decrease

Insert Lesson Title Here

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Percents can be used to describe a change. Percent change is the ratio of the amount of change to the original amount.

Percent increase describes how much the original amount increases.Percent decrease describes how much the original amount decreases.

amount of changeoriginal amountpercent change =

Find the percent increase or decrease from 16 to 12.

Additional Example 1: Percent Increase and Decrease

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This is percent decrease.16 – 12 = 4 First find the amount of change.

Think: What percent is 4 of 16?

amount of decreaseoriginal amount

416 Set up the ratio.

Additional Example 1 Continued

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= 25% Write as a percent.

= 0.25 Find the decimal form.416

From 16 to 12 is a 25% decrease.

Find the percent increase or decrease from 15 to 20.

Try This: Example 1

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This is percent increase.

20 – 15 = 5 First find the amount of change.


amount of increaseoriginal amount


Try This: Example 1 Continued

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= 0.25 Find the decimal form.520

From 15 to 20 is a 25% increase.

A. When Jim was exercising, his heart rate went from 70 beats per minute to 98 beats per minute. What was the percent increase?

Additional Example 2: Life Science Application

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28 70 Set up the ratio.



Additional Example 2 Continued

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= 0.4 Find the decimal form. 7028

Jim’s heart rate increased by 40% when he exercised.

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?

Additional Example 2B: Application

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1.25 – 0.85 = 0.40 First find the amount of change.

Think: What percent is 0.40 of 1.25?

amount of decreaseoriginal amount 1.25

0.40Set up the ratio.

Additional Example 2B Continued

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= 0.32 Find the decimal form. 1.250.40

The value of the stock decreased by 32%.

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?

Try This: Example 2A

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Think: What percent is 25 of 50?25




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= 0.5 Find the decimal form. 5025

The blinking of Jeff’s eyelids increased by 50% when he watched TV.

Try This: Example 2A Continued

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?

Try This: Example 2B

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9.00 – 3.80 = 5.20 First find the amount of change.

Think: What percent is 5.20 of 9.00?

amount of decreaseoriginal amount 9.00

5.20Set up the ratio.

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The value of the stock decreased by about 57.8%.

9.005.20 = 0.57 Find the decimal form.

= 57.7% Write as a percent.

Try This: Example 2B Continued

A. Sarah bought a DVD player originally priced at $450 that was on sale for 20% off. What was the sale price?

Additional Example 3A: Percent Increase and Decrease

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$450 20% First find 20% of $450.$450 0.20 = $9020% = 0.20The 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.

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?

Additional Example 3B: Percent Increase and Decrease

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$85 40% First find 40% of $85.$85 0.40 = $34 40% = 0.40The 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.

A. Lily bought a dog house originally priced at $750 that was on sale for 10% off. What was the sale price?

Try This: Example 3A

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$750 10% First find 10% of $750.$750 0.10 = $7510% = 0.10The 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.

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?

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$30 50% First find 50% of $30.$30 0.50 = $15 50% = 0.50The 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.

Try This: Example 3B

Lesson QuizFind each percent increase or decrease to the nearest percent.

1. from 12 to 15 2. from 1625 to 14003. from 37 to 1254. from 1.25 to 0.855. A computer game originally sold for $40 but is now on sale for 30% off. What is the sale price of the computer game?

14% decrease25% increase

Insert Lesson Title Here

238% increase32% decrease

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$28

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