Percentages

1
Percentages
Prior knowledge
Understand
How to find a fraction of a quantity by
cancelling factors
That multiplication is commutable, i.e. 2
3 6 and 3 2 6
2
Percentages
Finding a fraction of a quantity
Method 1 (non-calculator)
Find 4 of 20 5
or 4 20 5
Find 1 of 20 5
20 5 4
4

16
So 4 of 20 is 5
4
To do this in one step we can look at the question
Divide by 5 and show it by crossing out the 5 on
the bottom and cross out the 20 and replace with
4 (the answer to 20 5)
4
4 20 5
16
3
Percentages
Using this method to find percentages of a
quantity
Example 1
2
12 200 100
24
Find 12 of 200
cancel common factors
Example 2
40 270 100
108
Find 40 of 270
cancel common factors
Example 3
0.7
Find 35 of 14
35 14 100
4.9
cancel common factors
4
Percentages
Now do these 1. Find 10 of 250 2. Find 6
of 500 3. Find 25 of 24 4. Find 30 of
80 5. Find 15 of 4.20
10 250 100
25
6 500 100
30
6
25 24 100
6
30 80 100
24
21
15 420 100
63p
Convert to pence
5
Percentages
Now do these 1. Find 10 of 25 2. Find 5
of 700 3. Find 30 of 50 4. Find 15 of
70 5. Find 75 of 9.6
2.5
35
15
11.5
7.2
6
Percentages
Using a calculator we use the same method
Example 1
6.4 15.95 100
1.0208
Find 6.4 of 15.95
Press the buttons on your calculator as follows
Example 2
2.22
Find 8.6 of 25.84
6.4 25.84 100
Press the buttons on your calculator as follows
Notice how the answer is rounded to 2 d.p.
because the question is about money
7
Percentages
Now do these 1. Find 11.2 of 192.66 2.
Find 83 of 34 3. Find 26 of 62 4. Find
36.5 of 170 5. Find 16 of 120cm 6. Find
95 of 240g 7. Find 22 of 423 8. Find 12
of 19.6
21.58
28.22
16.12
63.05
19.2cm
228
93.06
2.35
8
Percentages
You will notice that whatever the percentage you
are finding the first step is always the
percentage 100
This calculation can be done in your head to gain
the percentage multiplier
e.g. to find 6.4 of 15.95 we first calculate 6.4
100
6.4 100 0.064
This means that we can calculate 6.4 of 15.95 by
pressing the following calculator buttons
1.0208
9
Percentages
The percentage multiplier is the percentage 100
Now do these just using the percentage
multiplier 1. Find 31.6 of 242.33 2. Find
73 of 56 3. Find 23 of 84 4. Find 42.3
of 240 5. Find 12 of 325cm
76.57628
40.88
19.32
101.52
39cm