~ GCSE Mathematics – Probability 1 ~
Page 1 of 2
Probability Scale
The probability that an event will happen is a number in the range from 0 (impossible) to 1 (certain).
impossible
certain
Place the following fractions and descriptions on the scale and think of an example of something for
each probability.
1
,
4
1
,
6
7
,
12
5
,
6
unlikely,
likely,
even chance, very likely
Definition: For equally likely outcomes:
probabilit y of a particular outcome 
number way s wanted outcome can happen
total number of possible outcomes
Exercise
Use the number square to help work out the following probabilities giving your answers in their
simplest forms.
A number is chosen at random between 1 and 100 (inclusive).
Work out the probability that the number is...
1. A multiple of 10
2. A multiple of 5
1
2
3
4
5
6
7
8
9
10
3. A multiple of 2
11
12
13
14
15
16
17
18
19
20
4. Greater than 90
21
22
23
24
25
26
27
28
29
30
5. Less than 5
31
32
33
34
35
36
37
38
39
40
6. Greater than 5
41
42
43
44
45
46
47
48
49
50
7. Between 20 and 30 (inclusive)
51
52
53
54
55
56
57
58
59
60
8. A two-digit number
61
62
63
64
65
66
67
68
69
70
9. A factor of 50
71
72
73
74
75
76
77
78
79
80
10. A factor of 24
81
82
83
84
85
86
87
88
89
90
11. A factor of 11
91
92
93
94
95
96
97
98
99
100
12. A multiple of 11
13. A square number
14. An odd number
15. A prime number
Mutually Exclusive outcomes
Outcomes are mutually exclusive if they cannot happen at the same time. Give examples of mutually
exclusive outcomes from the list above.
marian.harrington@abingdon-witney.ac.uk
~ GCSE Mathematics – Probability 1 ~
Page 2 of 2
Probabilities from two-way tables
1.
Survey of favourite TV channel from 80 adults:
BBC1
Male
8
Female
12
ITV
Ch4
total
7
20
28
total
a)
b)
c)
d)
e)
80
Complete the table.
How many people surveyed were female?
What percentage of people surveyed were female?
What’s the probability that a person selected at random prefers ITV?
What’s the probability that a man selected at random prefers BBC1?
f) How many people would you expect to prefer BBC1 out of a similar group of 400 adults?
2. Survey results of favourite subject from 106 children:
English
Girls
20
Boys
Total
a)
b)
c)
d)
e)
Maths
Science
13
total
50
15
38
40
106
Complete the table
How many boys surveyed preferred Science?
What percentage of the students surveyed were female?
What’s the probability that a student selected at random prefers Maths?
What’s the probability that a student selected at random does not prefer Maths?
f) How many people would you expect to prefer English out of a similar group of 500 students?
Probability of an event not happening
The sum of the probabilities of all mutually exclusive outcomes of an event is _______.
If the probability of an event happening is P, then the probability of it not happening is _______.
e.g. If the probability of it raining tomorrow is 0.7, what is the probability of it not raining
tomorrow?
Give other examples.
Additional practice: Edexcel 16+ book p. 263 Ex. 23A & p. 265 Ex 23B
marian.harrington@abingdon-witney.ac.uk