Section 3.9

PercentsMr. Beltz &Mr. Sparks

RatioA PERCENT is a ratio that compares a number

to 100.

You can write a percent as a FRACTION, DECIMAL, or a number followed by a % sign.

40100

.40

40% This is the way we normally write percents.

Changing Decimals to Percents

To change a decimal to a percent, move the decimal place two places to the RIGHT [same as multiplying by 100]

.4 = 40%

.04 = 4%

.004 = .4 %

1.2 = 120%

Changing a Fraction to a PercentTo change a Fraction to a Percent:

Divide the TOP by the BOTTOM

Numerator = 4Denominator 10

[Remember the TOP number goes into the Calculator 1st]

10/4 = .4

.4 = 40%

Try these Percents.80

.08

1.890

80100

45

Writing Equations for Percents

Objective: Write equations to solve problems involving percents.

Main Idea

We can write an equation to find the percent of a number. We will change the words of a statement into math symbols.

IS EQUALS [ = ]

Of MULTIPLY [ * ]

ExampleIS EQUALS [ = ] Of MULTIPLY

[ * ]

What number is 20% of 750?

x = 20 * 750

x = .20 * 750

x = 150

Example 2IS EQUALS [ = ] Of MULTIPLY [

* ]

What percent of 80 is 4?

x * 80 = 4

x * 80 = 4 80 80

x = .05 5%

Practice

P. 186

# 1-10, 13-1-27

*Show your work!

Sales TaxDiscount is a PERCENTAGE off of a price.

[Subtract]

Tax is a Percentage added to the price. [Add]

Example. Neiman Marcus has a sale. There is a 10% discount for all items purchased in the store. If you buy a coat for $100, how much do you have to pay?

10% off of a $100 coat is how much?

.10 * 100 = x

The discount is $10.

Regular price of the coat: $100 - $10 (discount) = $90

Example. You buy a coat at Neiman Marcus for $100. There is a 6% sales tax added to the price. How much does the coat cost?

$100 * .06 = $6

Original Price + Sales tax = final price

$100 + $6 = $106

Practice

P.186 #11,12.