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Percentage Change

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11

% Increase & Decrease

Finding a value after a percentage Increase and Decrease by a percentage
Finding the original given a value after a % Increase and Decrease

Khan Video Tutorial: Video 1 | Video 2 | Video 3 | Video 4 | Video 5

 

Class notes | pdf

Percentage Booklet | pdf

Application Problems p347 Ex 24.2 and p352 Ex 24.3

p36 Ex 3.03

NuLake p20, 21

 

Percentage Increase

Increase an amount by 28%

100% + 28% = 128%

So it's the same as 128% of the amount.

Multiply by 1.28

 

 

 

 

Question

Working

Answer

Increase $520 by 28%

$520 x 1.28 =

$665.60

Remember the 'shift =' for the % calculator function

Calculator use:
520 x 28 % +

 

 

 

 

Increase $8.40 by 65%

$8.40 x 1.65 =

$13.86

 

Calculator use:
8.4 x 65 % +

 

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Percentage Decrease

Decrease an amount by 28%

100% - 28% = 72%

So it's the same as 72% of the amount.

Multiply by 0.72

       

Question

Working

Answer

Decrease $520 by 28%

$520 x 0.72 =

$374.40

Remember the 'shift =' for the % calculator function

Calculator use:
520 x 28 % -

 

 

 

 

Decrease $8.40 by 65%

$8.40 x 0.35 =

$2.94

 

Calculator use:
8.4 x 35 % -

 

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Finding the Original Amount

Increasing by 10% then decreasing by 10% does NOT get back to the original
The percentages are on different amounts


eg Increase $400 by 10%

$400 x 1.1 = $440

The 10% is on the $400 = $40
We add on $40


Now decrese $440 by 10%


$440 x 0.9 = $396
NOT back to $400!


The 10% is on the $440 = $44
We take off $44

 

We must DIVIDE BACK to find the original amount

Increase an amount by 28%

100% + 28% = 128%

So it's the same as 128% of the amount.

Multiply by 1.28 to get the new amount

DIVIDE by 1.28 to get the ORIGINAL amount

       

Question

 

Working

Answer

A shirt was increased by 28% and it now costs $56
What was the original cost?

$original x 1.28 = $56
So $56 ÷ 1.28 =

$43.75

 

 

 

 

Petrol was increased by 8% and it now costs $1.43 per litre
What was the original cost?

$original x 1.08 = $1.43
So $1.43 ÷ 1.08 =

$1.32 (2dp)

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We must DIVIDE BACK to find the original amount

Decrease an amount by 28%

100% - 28% = 72%

So it's the same as 72% of the amount.

Multiply by 0.72 to get the new amount

DIVIDE by 0.72 to get the ORIGINAL amount

       

Question

 

Working

Answer

A shirt was decreased by 28% and it now costs $34
What was the original cost?

$original x 0.72 = $34
So $34 ÷ 0.72 =

$47.22

 

 

 

 

Chocolate fish were on sale for 15% off, and now costs $1.20 each
What was the cost before the sale?

$original x 0.85 = $1.20
So $1.20 ÷ 0.85 =

$1.41 (2dp)

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We must DIVIDE BACK to find the original amount

Finding 28% of something

Multiply by 0.72 to get the new amount

DIVIDE by 0.72 to get the ORIGINAL amount

       

Question

 

Working

Answer

28% of Farmer Bobs sheep are going to be sold. 154 sheep were sold. How many sheep were there alltogether?

Original x 0.28 = 154
So 154 ÷ 0.28 =

550 sheep

 

 

 

 

65% of apples produced by Farmer Bob are exported. There were 364 cartons exported.
How many cartons were NOT exported?

Original x 0.65 = 364
So 364 ÷ 0.65 = 560
560 cartons total
560 - 364 = NOT exported

196 cartons

 

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