Unit 10 Section 3 : Finding Probabilities Using Lists Of Outcomes
In Unit 10 Section 2 we saw three methods of listing all the outcomes when two or more experiments take place at the same time.
If the outcomes in each of these experiments are equally likely, then the combined outcomes will also be equally likely.
Example Question
When two unbiased coins are tossed, determine the probability of obtaining:
(a) two heads,
(b) two tails,
(c) a head and a tail.
One of the ways we can list all the possible outcomes is to use a table (or possibility space).
We can see that there are four possible outcomes in this situation: HH, HT, TH and TT.
Because the outcomes on each coin are equally likely, the four "combined" outcomes are also equally likely.
This means we can now use this formula to work out the probability of particular events:
(a) 
Probability of "two heads" = 
number of successful outcomes (those which have two heads) 
= 
1 


number of possible outcomes (from both experiments) 
4 
(b) 
Probability of "two tails" = 
number of successful outcomes (those which have two tails) 
= 
1 


number of possible outcomes (from both experiments) 
4 
(c) 
Probability of "a head and a tail" = 
number of successful outcomes (those with a head and a tail) 
= 
2 
= 
1 



number of possible outcomes (from both experiments) 
4 
2 
Practice Questions
Work out the answer to each of these questions then click on the button marked
to see whether you are correct.
Practice Question 1
Caitlin and Dave each buy a chocolate bar from a vending machine that sells Aero, Bounty, Crunchie and Dime bars.
What is the probability that they both choose the same type of chocolate bar?.
In the previous section we systematically listed all the possible outcomes in this situation.
The list of possible outcomes is shown on the right.
Because Caitlin and Dave are choosing randomly, each chocolate bar is equally likely to be chosen.
Each of the combined outcomes shown on the right is also therefore equally likely.
(a) In how many outcomes did Caitlin and Dave choose the same chocolate bar?
(b) How many possible outcomes are there in this situation?
(c) What is the probability that they choose the same type of chocolate bar?


Practice Question 2
Two fair dice are rolled at the same time and the scores are added together.
A table can be used to find the 36 possible outcomes.
The outcomes on each dice are equally likely, so these 36 combined outcomes will also be equally likely.
(a) What is the probability of obtaining a score of 6?
(b) What is the probability of obtaining a score greater than 9?
(c) What is the probability of obtaining a score less than 7?
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.
You will need some paper and something to write with for the next two questions.
You should use the method specified in each question to work out possible outcomes.
You can then type in the answers to each question in the boxes below and check them.


You have now completed Unit 10 Section 3
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