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Objective

Compute mentally with percents

Example 1 Use Fractions to Compute Mentally

Example 2 Use Fractions to Compute Mentally

Example 3 Use Decimals to Compute Mentally

Example 4 Use Decimals to Compute Mentally

Example 5 Use Proportion to Solve a Problem

Compute 40% of 80 mentally.

4 8

Write the problem40% of 80Remember “of” means multiplication

00

Bring down the zeros

Remember % means 100 so to take away the % sign divide by 100

3200100

Answer: 32

1/5

Multiply the 4 832

Since there are 2 zeros in both numerator and denominator, cancel them out

32

Compute 20% of 60 mentally.

Answer: 12

1/5

Compute of 75 mentally.

70%

Write problem66 % of 752

3 Round to easy number

70

2/5

is about 70%

75 is about 70 or 80 (either will work)

Remember: of means to multiply

Compute of 75 mentally.

70%

66 % of 7523

70

2/5

Multiply the 7 7

7 749

Bring down the zeros

00

Remember % means 100 so to take away the % sign divide by 100

100 Since there are 2 zeros in both numerator and denominator, cancel them out49Answer:

Answer: 210

Compute of 300 mentally.

2/5

Compute 10% of 65 mentally.

3/5

Write problem10% of 65

Round to easy number

10% is already an easy number

10%

Remember: of means to multiply

10%

65 is about 60 or 70 (either will work)

70

Compute 10% of 65 mentally.

3/5

10% of 65

10% 10% 70

Multiply the 1 7

1 7

Bring down the zeros

700

Remember % means 100 so to take away the % sign divide by 100

100Since there are 2 zeros in both numerator and denominator, cancel them out

7Answer:

Answer: 1

Compute 10% of 13 mentally.

3/5

Compute 1% of 304 mentally.

4/5

Write problem1% of 304

Round to easy number

1% is already an easy number

1% 1%

Remember: of means to multiply

304 is about 300

300

Answer:

Compute 1% of 304 mentally.

4/5

1% of 304

1% 1% 300

Multiply the 1 3

1 33

Bring down the zeros

00Remember % means 100 so to take away the % sign divide by 100100Since there are 2 zeros in both numerator and denominator, cancel them out

3

Answer: 2.5

Compute 1% of 244 mentally.

4/5

TECHNOLOGY A company produces 2,500 of a particular printer. They later discover that 25% of the printers have defects. How many printers from this group have defects?

Remember: Part = % Whole

5/5

Define the variable: How many printers are defective

Since all the printers are not defective, need to find the part

x

How many printers from this group have defects

Convert 25% to a decimal

x = 0.25

TECHNOLOGY A company produces 2,500 of a particular printer. They later discover that 25% of the printers have defects. How many printers from this group have defects?

Company produces 2,500 which is the total number of printers

5/5

x

How many printers from this group have defects

x = 0.25x = 0.25 2,500

Since the variable is by itself, multiply 0.25 2,500

x = 625

Add dimensional analysis

x = 625 defective printers

Answer: 625 printers had defects

TECHNOLOGY A company produces 1,400 of a particular monitor. They later discover that 20% of the monitors have defects. How many monitors from this group have defects?

Answer: 280 defective monitors

*

5/5