Probability
OCR Stage 6
1
Probability
• In an experiment, things that can
happen are called possible outcomes
• Each of these has a possibility
2
Probability of an event happening
• Probability = Required Outcome
Total Possible Outcomes
• i.e. The number of required
outcomes divided by the total
number of possible outcomes
3
Getting a 4 on a single dice
• There is 1 required outcome – a four
• There are 6 possible outcomes
• P(4) =
1
6
The P says, “The
probability of”
The answer may
be written as a
fraction, decimal
or percentage
NEVER as
“one out of six”
4
The probability of not
getting a 4?
• There are five numbers on the
single dice that are not a 4
• There are six possible outcomes
• So, P(Not getting a 4) = 5

6
P 4
5
It is important that you notice
that the two probabilities add
together to give 1
• The total probabilities of any
event have a total of
1
• 1/6 + 5/6 = 1
Probability of getting
a 4 on a single dice
Probability of not
getting a 4 on a single
dice
6
Sum of all probabilities equal 1.
Example
The probability of a football team’s
results in the next match are:
• P(Win) = 0.3
• P(Lose) = 0.5
• What is the probability of a draw?
7
Remember the total of all
the probabilities is 1
The team must win, lose or draw.
• P(Win) + P(Lose) + P(Draw) = 1
This is 0.3
This is 0.5
• 0.3 + 0.5 + P(Draw) = 1
• 0.8 + P(Draw) = 1
• P(Draw) = 0.2
8
Mutually Exclusive events
• Can NOT happen at the same time
• Examples
– A HEAD and a TAIL when coin is spun
– A 6 and an odd number when die rolled
– A level 5 and a level 6 on a SAT paper
9
Example
•
•
•
•
•
•
•
Sam and Pete play chess
P(Sam wins) = 0.4
P(Pete wins) = 0.3
What is P(Draw) ?
0.4 + 0.3 + P(Draw) = 1
0.7 + P(Draw) = 1
P(Draw) = 0.3
10
Exercises – to be
completed next lesson
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OCR Stage 5/6 Text,
Stage 6, Chapter 4, Page 182
Ex 4.1A
Ex 4.1B
Ex 4.2A
Ex 4.2B
11