Pre-Algebra 8-3 Finding a Number When the Percent Is Known 8-3 Finding a Number When the Percent Is...

Pre-Algebra

8-3 Finding a Number When the Percent Is Known8-3 Finding a Number When the

Percent Is Known

Pre-Algebra

Warm UpWarm Up

Problem of the DayProblem of the Day

Lesson PresentationLesson Presentation

Pre-Algebra

8-3 Finding a Number When the Percent Is Known

Warm Up

1. What percent of 20 is 18?

2. What percent of 400 is 50?

3. 9 is what percent of 27?

4. 25 is what percent of 4?

90%

12.5%

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625%

331

3 %

Pre-Algebra


Problem of the Day

The original price of a sweater is $40. The sweater goes on sale for 75% of its original price. Later, the sweater goes on clearance for 50% of its sale price. What is the clearance price of the sweater?$15

Pre-Algebra


Learn to find a number when the percent is known.

Pre-Algebra


The Pacific giant squid can grow to a weight of 2000 pounds. This is 1250% of the maximum weight of the Pacific giant octopus. When one number is known, and is relationship to another number is given by a percent, the other number can be found.

Pre-Algebra


60 = 0.12n

60 is 12% of what number?

Additional Example 1: Finding a Number When the Percent Is Known

Set up an equation to find the number.

60 = 12% n

= n 60

0.12 0.12

0.12Divide both sides by 0.12.

500 = n

60 is 12% of 500.

Set up an equation.

1210012% =

Pre-Algebra


75 = 0.25n

75 is 25% of what number?

Try This: Example 1

Set up an equation to find the number.

75 = 25% n

= n 75

0.25 0.25

0.25 Divide both sides by 0.25.

300 = n

75 is 25% of 300.

Set up an equation.

2510025% =

Pre-Algebra


Anna earned 85% on a test by answering 17 questions correctly. If each question was worth the same amount, how many questions were on the test?

Additional Example 2: Application

Choose a method: Set up a proportion to find the number.

Think: 85 is to 100 as 17 is to what number?

= 85

10017

nSet up a proportion.

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Additional Example 2 Continued

There were 20 questions on the test.

85 n = 100 17 Find the cross products.

85 n = 1700

n = Solve for n.1700

85

n = 20

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Try This: Example 2

Choose a method: Set up a proportion to find the number.


= 80

10020

nSet up a proportion.

Tom earned 80% on a test by answering 20 questions correctly. If each question was worth the same amount, how many questions were on the test?

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Try This: Example 2 Continued

There were 25 questions on the test.

80 n = 100 20 Find the cross products.

80 n = 2000

n = Solve for n.2000

80

n = 25

Pre-Algebra


A. A fisherman caught a lobster that weighed 11.5 lb. This was 70% of the weight of the largest lobster that fisherman had ever caught. What was the weight, to the nearest tenth of a pound, of the largest lobster the fisherman had ever caught?

Additional Example 3A: Life Science Application

Choose a method: Set up an equation.

Think: 11.5 is 70% of what number?

11.5 = 70% n Set up an equation.

Pre-Algebra


Additional Example 3A Continued

16.4 n

The largest lobster the fisherman had ever caught was about 16.4 lb.

11.5 = 0.70 n 70% = 0.70

Solve for n. 11.5 0.70

n=

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B. When a giraffe is born, is approximately 55% as tall as it will be as an adult. If a baby giraffe is 5.2 feet tall when it is born, how tall will it be when it is full grown, to the nearest tenth of a foot?

Additional Example 3B: Life Science Application

Choose a method: Set up a proportion.

Think: 55 is to 100 and 5.2 is to what number?

= h

5.2 55 100

Set up a proportion.

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Additional Example 3B Continued

55h = 520

This giraffe will be approximately 9.5 feet tall when full grown.

55 h = 100 5.2 Find the cross products.

52055

h = Solve for h.

h 9.5

Pre-Algebra


A. Amy caught a fish that weighed 15.5 lb. This was 85% of the weight of the largest fish that Amy had ever caught. What was the weight, to the nearest tenth of a pound, of the largest fish that Amy had ever caught?

Try This: Example 3

Choose a method: Set up an equation.

Think: 15.5 is 85% of what number?

15.5 = 85% n Set up an equation.

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18.2 n

The largest fish that Amy had ever caught was about 18.2 lb.

15.5 = 0.85 n 85% = 0.85

Solve for n. 15.5 0.85

n=

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B. When Bart was 12, he was approximately 85% of the weight he is now. If Bart was 120 lb, how heavy is he now, to the nearest tenth of a pound?

Try This: Example 3

Choose a method: Set up a proportion.


= w

120 85 100

Set up a proportion.

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85w = 12000

Bart weighs approximately 141.2 lbs.

85 w = 100 120 Find the cross products.

12000 85

w = Solve for h.

w 141.2


Pre-Algebra


Three Types of Percent Problems

1. Finding the percent of a number. 15% of 120 = n

2. Finding the percent one number is of another.p% of 120 = 18

3. Finding a number when the percent is known.15% of n = 18

You have now seen all three types of percent problems.

Pre-Algebra


Lesson Quiz

1. 10 is 12 % of what number?

2. 326 is 25% of what number?

3. 44% of what number is 11?

4. 290% of what number is 145?

5. Larry has 9 novels about the American

Revolutionary War. This represents 15% of his

total book collection. How many books does Larry

have in all?

1304

80

25

50

12

60

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