Unit 21 Section 6 : Addition Law for Mutually Exclusive Events
Two events are mutually exclusive if only one can happen in any given experiment.
Example 1
A bag contains red balls and yellow balls.
When a ball is taken out it is either red or yellow, but it cannot be both.
The events 'red ball' and 'yellow ball' are therefore mutually exclusive.
Addition Law for Mutually Exclusive Events
If there are two events, A and B, such that
the probability of A happening is P(A) and the probability of B happening is P(B),
and if A and B are mutually exclusive, then:
P(A or B) = P(A) + P(B)

Example 2 The probability of my team winning is 
2 
and the probability of them getting a draw is 
5 
for a particular match.   
9 
9 
The two events win and draw are mutually exclusive, so to work out the probability of winning or drawing, we add the probabilities together:
P(win OR draw) = P(win) + P(draw) = 
2 
+ 
5 
= 
7    
9 
9 
9 
Practice Question
A spinner has five possible outcomes A, E, O, P and S.
The probabilities of each outcome are shown in the table below.
Exercises
Work out the answers to the questions below and fill in the boxes. Click on the
button to find out whether you have answered correctly. If you are right
then will appear and you should move on to the next
question. If appears then your answer is wrong. Click
on to clear your original answer and have another go.
If you can't work out the right answer then click on
to see
the answer.
Make sure you simplify all the fractions in your answers.
You have now completed Unit 21 Section 6
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