© boardworks ltd 2005 1 of 56 n5 percentages ks4 mathematics
TRANSCRIPT
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N5.1 Fractions, decimals and percentages
Contents
N5 Percentages
N5.5 Reverse percentages
N5.6 Compound percentages
N5.2 Percentages of quantities
N5.4 Increasing and decreasing by a percentage
N5.3 Finding a percentage change
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Writing percentages as fractions
‘Per cent’ means ‘out of 100’.
To write a percentage as a fraction we write it over a hundred.
For example,
46% =46
100Cancelling:
46100
=23
50
2350
180% =180100
Cancelling:180100
=
9
5
95
= 1 45
7.5% =7.5100
Cancelling:15
200
3
40
=340
=15
200
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Writing percentages as decimals
We can write percentages as decimals by dividing by 100.
For example,
46% =46
100= 46 ÷ 100 = 0.46
7% =7
100= 7 ÷ 100 = 0.07
130% =130100
= 130 ÷ 100 = 1.3
0.2% =0.2
100= 0.2 ÷ 100 = 0.002
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Writing fractions as percentages
To write a fraction as a percentage, we can find an equivalent fraction with a denominator of 100.
85
For example,
=1720 100
× 5
× 5
and =10085
85%
1 725
= =3225
× 4
100
× 4
128and =
100128
128%
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To write a fraction as a percentage you can also multiply it by 100%.
For example,38
=38
× 100%
=3 × 100%
8
25
2
=75%
2
= 3712%
Writing fractions as percentages
Remember, multiplying by 100% does not change the value of the number because it is equivalent to multiplying by 1.
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Writing decimals as percentages
Decimals can also be converted to percentages by multiplying them by 100%.
For example,
0.08 = 0.08 × 100%
= 8%
1.375 = 1.375 × 100%
= 137.5%
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Using a calculator
We can also convert fractions to decimals and percentages using a calculator.
For example,
516
= 5 ÷ 16 × 100% = 31.25%
47
= 4 ÷ 7 × 100% = 57.14% (to 2 d.p.)
58
=1 138
= 13 ÷ 8 × 100% = 162.5%
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One number as a percentage of another
There are 35 sweets in a bag. Four of the sweets are orange flavour.
What percentage of sweets are orange flavour?
Start by writing the proportion of orange sweets as a fraction.
4 out of 35 =435
Then convert the fraction to a percentage.
× 100% =435
4 × 100%35
20
7=
80%7
= 1137%
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Petra put £32 into a bank account. After one year she received 80p interest.
To write 80p out of £32 as a fraction we must use the same units.
In pence, Petra gained 80p out of 3200p.
803200
=1
40
We then convert the fraction to a percentage.
140
× 100% = 100%
40
5
2
= 2.5%
One number as a percentage of another
What percentage interest rate did she receive?
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N5.2 Percentages of quantities
Contents
N5 Percentages
N5.5 Reverse percentages
N5.1 Fractions, decimals and percentages
N5.6 Compound percentages
N5.4 Increasing and decreasing by a percentage
N5.3 Finding a percentage change
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Calculating percentages using fractions
Remember, a percentage is a fraction out of 100.
15% of 90, means “15 hundredths of 90”
or
15100
× 90 =15 × 90
100
3
20
9
2
= 272
= 13 12
Find 15% of 90
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Calculating percentages using decimals
We can also calculate percentages using an equivalent decimal operator.
4% of 9 = 0.04 × 9
= 4 × 9 ÷ 100
= 36 ÷ 100
= 0.36
What is 4% of 9?
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Estimating percentages
We can find more difficult percentages using a calculator.
It is always sensible when using a calculator to start by making an estimate.
For example, estimate the value of:
19% of £82 20% of £80 = £16
27% of 38m 25% of 40m =10m
73% of 159g 75% of 160g = 120g
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Using a calculator
One way to work out a percentage using a calculator is by writing the percentage as a decimal. For example,
What is 38% of £65?
38% = 0.38
So we key in:
0 . 3 8 × 6 5 =
The calculator will display the answer as 24.7.
We write the answer as £24.70
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We can also work out a percentage using a calculator by converting the percentage to a fraction. For example,
What is 57% of £80?
57% = 57100
= 57 ÷ 100So we key in:
The calculator will display the answer as 45.6
We write the answer as £45.60
5 7 ÷ 1 0 0 × 8 0 =
Using a calculator
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We can also work out percentages on a calculator by finding 1% first and then multiplying by the required percentage.
What is 37.5% of £59?
1% of £59 is £0.59 so, 37.5% of £59 is £0.59 × 37.5.
We key in:
And get an answer of 22.125
We write the answer as £22.13 (to the nearest penny).
0 . 5 9 × 3 7 . 5 =
Using a calculator
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N5.3 Finding a percentage change
Contents
N5.5 Reverse percentages
N5 Percentages
N5.1 Fractions, decimals and percentages
N5.6 Compound percentages
N5.2 Percentages of quantities
N5.4 Increasing and decreasing by a percentage
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Sometimes, we are given an original value and a new value and we are asked to find the percentage increase or decrease.
Finding a percentage increase or decrease
We can do this using the following formulae:
Percentage increase =actual increase
original amount× 100%
Percentage decrease =actual decrease
original amount× 100%
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Finding a percentage increase
The actual increase = 4.2 kg – 3.5 kg
= 0.7 kg
The percentage increase =0.73.5
× 100%
= 20%
A baby weighs 3.5 kg at birth. After 6 weeks the baby’s weight has increased to 4.2 kg.
What is the baby’s percentage increase in weight?
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Finding a percentage decrease
All t-shirts were £25 now
only £17!
What is the percentage decrease?
The actual decrease = £25 – £17 = £8
The percentage decrease =825
× 100% = 32%
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Finding a percentage profit
Her actual profit = 50p – 32p
= 18p
Her percentage profit =1832
× 100%
= 56.25%
A shopkeeper buys chocolate bars wholesale at a price of 32p per bar. She then sells the chocolate bar in her shop at 50p each.
What is her percentage profit?
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Finding a percentage loss
Her actual loss = £3.68 – £3.22
= 46p
Her percentage loss =0.463.68
× 100%
= 12.5%
A share dealer buys a number of shares at £3.68 each. After a week the price of the shares has dropped to £3.22.
What is her percentage loss?
Make sure the units are the same.
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N5.4 Increasing and decreasing by a percentage
Contents
N5.5 Reverse percentages
N5 Percentages
N5.1 Fractions, decimals and percentages
N5.6 Compound percentages
N5.2 Percentages of quantities
N5.3 Finding a percentage change
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Percentage increase
There are two methods to increase an amount by a given percentage.
The value of Frank’s house has gone up by 20% in three years. If the house was worth £150 000
three years ago, how much is it worth now?
Method 1
We can work out 20% of £150 000 and then add this to the original amount.
= 0.2 × £150 000= £30 000
The amount of the increase = 20% of £150 000
The new value = £150 000 + £30 000= £180 000
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Percentage increase
We can represent the original amount as 100% like this:
100%
When we add on 20%,
20%
we have 120% of the original amount.
Finding 120% of the original amount is equivalent to finding 20% and adding it on.
Method 2
If we don’t need to know the actual value of the increase we can find the result in a single calculation.
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Percentage increase
So, to increase £150 000 by 20% we need to find 120% of £150 000.
120% of £150 000 = 1.2 × £150 000
= £180 000
In general, if you start with a given amount (100%) and you increase it by x%, then you will end up with (100 + x)% of the original amount.
In general, if you start with a given amount (100%) and you increase it by x%, then you will end up with (100 + x)% of the original amount.
To convert (100 + x)% to a decimal multiplier we have to divide (100 + x) by 100. This is usually done mentally.
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Here are some more examples using this method:
Increase £50 by 60%.
160% × £50 = 1.6 × £50
= £80
Increase £24 by 35%
135% × £24 = 1.35 × £24
= £32.40
Percentage increase
Increase £86 by 17.5%.
117.5% × £86 = 1.175 × £86
= £101.05
Increase £300 by 2.5%.
102.5% × £300 =1.025 × £300
= £307.50
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Percentage decrease
There are two methods to decrease an amount by a given percentage.
A CD walkman originally costing £75 is reduced by 30% in a sale. What is the sale price?
Method 1We can work out 30% of £75 and then subtract this from the original amount.
= 0.3 × £75= £22.50
30% of £75 The amount taken off =
The sale price = £75 – £22.50= £52.50
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Percentage decrease
100%
When we subtract 30%
30%
we have 70% of the original amount.
70%
Finding 70% of the original amount is equivalent to finding 30% and subtracting it.
We can represent the original amount as 100% like this:
Method 2
We can use this method to find the result of a percentage decrease in a single calculation.
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Percentage decrease
So, to decrease £75 by 30% we need to find 70% of £75.
70% of £75 = 0.7 × £75
= £52.50
In general, if you start with a given amount (100%) and you decrease it by x%, then you will end up with (100 – x)% of the original amount.
In general, if you start with a given amount (100%) and you decrease it by x%, then you will end up with (100 – x)% of the original amount.
To convert (100 – x)% to a decimal multiplier we have to divide (100 – x) by 100. This is usually done mentally.
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Here are some more examples using this method:
Percentage decrease
Decrease £320 by 3.5%.
96.5% × £320 = 0.965 × £320
= £308.80
Decrease £1570 by 95%.
5% × £1570 = 0.05 × £1570
= £78.50
Decrease £65 by 20%.
80% × £65 = 0.8 × £65
= £52
Decrease £56 by 34%
66% × £56 = 0.66 × £56
= £36.96
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N5.5 Reverse percentages
Contents
N5 Percentages
N5.1 Fractions, decimals and percentages
N5.6 Compound percentages
N5.2 Percentages of quantities
N5.4 Increasing and decreasing by a percentage
N5.3 Finding a percentage change
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Reverse percentages
Sometimes, we are given the result of a given percentage increase or decrease and we have to find the original amount.
I bought some jeans in a sale. They had 15% off and I only paid £25.50 for them.
What is the original price of the jeans?
We can solve this using inverse operations.
Let p be the original price of the jeans.
p × 0.85 = £25.50 so p = £25.50 ÷ 0.85 = £30
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Sometimes, we are given the result of a given percentage increase or decrease and we have to find the original amount.
I bought some jeans in a sale. They had 15% off and I only paid £25.50 for them.
What is the original price of the jeans?
We can show this using a diagram:
Price before discount.
× 0.85%Price after discount.
÷ 0.85%
Reverse percentages
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Reverse percentages
We can also use a unitary method to solve these type of percentage problems. For example,
Christopher’s monthly salary after a 5% pay rise is £1312.50. What was his original salary?
The new salary represents 105% of the original salary.
105% of the original salary = £1312.50
1% of the original salary = £1312.50 ÷ 105
100% of the original salary = £1312.50 ÷ 105 × 100
= £1250This method has more steps involved but may be easier to remember.
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AN5.6 Compound percentages
Contents
N5.5 Reverse percentages
N5 Percentages
N5.1 Fractions, decimals and percentages
N5.2 Percentages of quantities
N5.4 Increasing and decreasing by a percentage
N5.3 Finding a percentage change
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A jacket is reduced by 20% in a sale.
Compound percentages
Two weeks later the shop reduces the price by a further 10%.
What is the total percentage discount?
When a percentage change is followed by another percentage change do not add the percentages together to find the total percentage change.
The second percentage change is found on a new amount and not on the original amount.
It is not 30%!
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Compound percentages
To find a 10% decrease we multiply by 90% or 0.9.
A 20% discount followed by a 10% discount is equivalent to multiplying the original price by 0.8 and then by 0.9.
To find a 20% decrease we multiply by 80% or 0.8.
original price × 0.8 × 0.9 = original price × 0.72
A jacket is reduced by 20% in a sale.
Two weeks later the shop reduces the price by a further 10%.
What is the total percentage discount?
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Compound percentages
This is equivalent to a 28% discount.
The sale price is 72% of the original price.
A 20% discount followed by a 10% discount
A 28% discount
A 20% discount followed by a 10% discount
A 28% discount
A jacket is reduced by 20% in a sale.
Two weeks later the shop reduces the price by a further 10%.
What is the total percentage discount?
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Compound percentages
After a 20% discount it costs 0.8 × £100 = £80
Suppose the original price of the jacket is £100.
After an other 10% discount it costs 0.9 × £80 = £72
£72 is 72% of £100.
72% of £100 is equivalent to a 28% discount altogether.
A jacket is reduced by 20% in a sale.
Two weeks later the shop reduces the price by a further 10%.
What is the total percentage discount?
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Jenna invests in some shares.
Compound percentages
After one week the value goes up by 10%.
The following week they go down by 10%.
Has Jenna made a loss, a gain or is she back to her original investment?
To find a 10% increase we multiply by 110% or 1.1.
To find a 10% decrease we multiply by 90% or 0.9.
original amount × 1.1 × 0.9 = original amount × 0.99
Fiona has 99% of her original investment and has therefore made a 1% loss.
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Jack puts £500 into a savings account with an annual compound interest rate of 6%.
Compound interest
How much will he have in the account at the end of 4 years if he doesn’t add or withdraw any money?
At the end of each year interest is added to the total amount in the account. This means that each year 5% of an ever larger amount is added to the account.
To increase the amount in the account by 5% we need to multiply it by 105% or 1.05.
We can do this for each year that the money is in the account.
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At the end of year 1 Jack has £500 × 1.05 = £525
Compound interest
At the end of year 2 Jack has £525 × 1.05 = £551.25
At the end of year 3 Jack has £ 551.25 × 1.05 = £578.81
At the end of year 4 Jack has £578.81 × 1.05 = £607.75
(These amounts are written to the nearest penny.)
We can write this in a single calculation as
£500 × 1.05 × 1.05 × 1.05 × 1.05 = £607.75
Or using index notation as
£500 × 1.054 = £607.75
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How much would Jack have after 10 years?
Compound interest
After 10 years the investment would be worth
£500 × 1.0510 = £814.45 (to the nearest 1p)
How long would it take for the money to double?
£500 × 1.0514 = £989.97 (to the nearest 1p)
£500 × 1.0515 = £1039.46 (to the nearest 1p)
Using trial and improvement,
It would take 15 years for the money to double.
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We can use powers to help solve many problems involving repeated percentage increase and decrease. For example,
Repeated percentage change
The population of a village increases by 2% each year.If the current population is 2345, what will it be in 5 years?
To increase the population by 2% we multiply it by 1.02.
After 5 years the population will be
2345 × 1.025 = 2589 (to the nearest whole)
What will the population be after 10 years?
After 5 years the population will be
2345 × 1.0210 = 2859 (to the nearest whole)
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Repeated percentage change
The car costs £24 000 in 2005. How much will it be worth in 2013?
To decrease the value by 15% we multiply it by 0.85.
After 8 years the value of the car will be
£24 000 × 0.858 = £6540 (to the nearest pound)
The value of a new car depreciates at a rate of 15% a year.
There are 8 years between 2005 and 2013.