2­10­11.notebook
February 10, 2011
2-10-11
Aim: SWBAT find the percent of increase or decrease.
Do Now: Check hw with key
HW: Pg. 368 # 3-13 odd
Test Tuesday
Aim/ HW/ Do Now
Feb 10­10:43 AM
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2­10­11.notebook
February 10, 2011
Feb 10­10:43 AM
To find Percent of Change (increase or decrease) set up
a proportion:
amount of increase or decrease
original number
=
r
100
** Read the question to decide whether to
use an amount of increase or decrease.
Notes/ Formula
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2­10­11.notebook
February 10, 2011
Find the percent of change: (Round decimal answers to the
nearest tenth of a percent)
1) What is the percent increase from $800 to $835
to the nearest 0.1%?
r
amount of increase
original number
= 100
Amount of increase
the price increased from
$800 to $835.
To find the amount of increase, subtract:
835 - 800 = 35
The amount of increase is $35.
Original number
the original price
The original price was $800.
r
we are solving for the percent
35
800
=
=
800 r
r
= 100
800r
800
r
35 100
3500
800
= 4.375
R ≈ 4.4%
The percent increase is
about 4.4%.
Practice #1
2) What is the percent decrease from $420 to $380
to the nearest 0.1%?
amount of decrease
original number
=
r
100
Amount of decrease
the price decreased from
$420 to $380
To find the amount of decrease, subtract:
420 - 380 = 40
The amount of decrease is $40.
Original number
the original price
The original price was $420.
r
we are solving for the percent
40
420
=
420 r
r
100
420r
420
r
=
=
40 100
4000
420
= 9.5238 . . .
R ≈ 9.5%
The percent decrease
is about 9.5%.
Practice #2
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February 10, 2011
3) The number of students in the math club increased
from 20 to 26. What was the percent increase?
(round your answer to the nearest 0.1%.)
r
amount of increase
original number
= 100
Amount of increase
the number of students
increased from 20 to 26.
To find the amount of increase, subtract:
26 - 20 = 6
The amount of increase is 6 students.
Original number
the original # of students
The original # of students was 20.
r
we are solving for the percent
6
20
=
r
100
20 r
20r
20
r
=
=
6 100
600
20
= 30
R = 30%
The percent increase
is 30%.
Practice #3
You Try!
Formula:
amount of or
original number
1) $50,000 to $40,000
amount of
original #
=
r
100
=
(decrease)
r
100
10,000
50,000
r
100
=
r = 20
R = 20% decrease
2) 600 pounds to 550 pounds (decrease)
amount of
original #
=
r
100
50
600
=
r
100
r = 8.333 . . .
R ≈ 8.3% decrease
3) 120 pages to 200 pages (increase)
amount of
original #
=
r
100
80
120
=
r
100
r = 66.666 . . .
R ≈ 66.7% increase
You Try!
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February 10, 2011
Find the new amount.
65 is decreased by 30%. (Two Steps!)
Step 1: Find the amount of decrease.
P
= 30
65 100
100P = 1950
100
100
P = 19.5
Step 2: Subtract the amount of decrease from the
original amount.
65 - 19.5 = 45.5
Feb 6­1:08 AM
Find the new amount.
26,000 is increased by 14.6%. (Two Steps!)
Step 1: Find the amount of increase.
P
100P = 379,600
= 14.6
26,000
100
100
100
P = 3796
Step 2: Add the amount of increase to the original
amount.
26,000 + 3796 = 29,796
Feb 6­1:08 AM
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