Section 4-3: Proportions and Percent
SPI 12F: Select ratios and proportions to represent real-world problems
Objectives:
• use proportions when solving percent problems
Vocabulary:
Percent: ratio that compares a number to 100
23% = 23
or n = part
100
100 whole
0%
10
This model shows
That 30 is 75% of 40.
10
10
75%
100%
10
30 (part)
40 (whole)
Solving Equations involving Percent
Find the Percent
What % of 80 is 18?
? = 18 part (is)
100
80 whole (of)
x
What
Find the Part
∙
of
80
=
is
80x = 18(100)
x = 22.5
18(100)
(Asking for percent)
Find 75% of 320.
75
100
= ? part (is)
320 whole (of)
.75
∙
% to decimal of
320
=
is
x
100x = 75(320)
x = 240
240 = x
Solving Equations involving Percent
Find the Whole
71% of what number is 87?
71
100
= 87 part (is)
x whole (of)
71x = 87(100)
x = 122.5
According to a survey, surface water accounts for 77.6% of
our country’s water supply. The surface water supply is
about 264.4 billion gallons per day. Find the total water
supply.
77.6
100
= 264.4
x
part (is)
whole (of)
77.6x = 264.4 (100)
x = 340.7
Solve Percents by translating into Equations
What percent of 170 is 68?
x
∙ 170 = 68
x = .4
convert to %: .4 x 100 = 40%
What is 85% of 320?
x
= .85 ∙ 320
x = 272
Real-world
Jane lives in Florida andplans to buy a
car that costs $13,500, plus 7.2% sales
tax. Her friend Julie lives in Georgia.
She also plans to buy a car for
$13,500, plus 5% sales tax. How much
more will Jane pay in sales tax?
Jane’s Cost for sales tax
Julie’s Cost for sales tax
Write a what, is, of statement. Write a what, is, of statement.
7.2% of 13,500 is what?
Translate to an Equation
0.072 ∙ 13,500 = x
972 = x
5% of 13,500 is what?
Translate to an Equation
0.05 ∙ 13,500 = y
675 = y
Answer the question: How much more did Jane pay?
972 – 675 = $297