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Calculating probabilities

Calculating probabilities

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Tutor: Bilal

Summary

Calculating probabilities

​​In a nutshell

Probabilities describe the likelihood of an event occurring. They are quantities that are always between 00 and 11.



Definitions

Here are some definitions that you need to know.


term

definition

Experiment

A repeatable process.

Event

One or more possible outcomes of an experiment.

Sample space

The set of all possible outcomes of an experiment.



Representing sample space

The sample space of an experiment can be represented with a sample space diagram. These are an easy way to list possible outcomes.


Example 1

A fair coin is tossed 33 times. What is the sample space of this experiment?


Let HH​ represent heads, and TT​ represent tails.

The set of possible options is:

{HHH,HHT,HTH,HTT,THH,THT,TTH,TTT}\underline{\{HHH,HHT,HTH,HTT,THH,THT,TTH,TTT\}}​​


Example 2

Two fair dice are tossed, and the numbers that they land on are multiplied together. What is the probability that the two numbers multiply to give 1212?


Draw a table to represent the sample space:



die 1

die 2

×\bf \times​​

1\bf1​​

2\bf2​​

3\bf3​​

4\bf4​​

5\bf5​​

6\bf6

1\bf1​​

11​​

22​​

33​​

44​​

55​​

66​​

2\bf2​​

22​​

44​​

66​​

88​​

1010​​

1212​​

3\bf3​​

33​​

66​​

99​​

1212​​

1515​​

1818​​

4\bf4​​

44​​

88​​

1212​​

1616​​

2020​​

2424​​

5\bf5​​

55​​

1010​​

1515​​

2020​​

2525​​

3030​​

6\bf6​​

66​​

1212​​

1818​​

2424​​

3030​​

3636​​

Identify the desired events in the sample space:




die 1

die 2

×\bf \times​​

1\bf1​​

2\bf2​​

3\bf3​​

4\bf4​​

5\bf5​​

6\bf6

1\bf1​​

11​​

22​​

33​​

44​​

55​​

66​​

2\bf2​​

22​​

44​​

66​​

88​​

1010​​

12\large\textcircled{\small12}​​

3\bf3​​

33​​

66​​

99​​

12\large\textcircled{\small12}​​

1515​​

1818​​

4\bf4​​

44​​

88​​

12\large\textcircled{\small12}​​

1616​​

2020​​

2424​​

5\bf5​​

55​​

1010​​

1515​​

2020​​

2525​​

3030​​

6\bf6​​

66​​

12\large\textcircled{\small12}​​

1818​​

2424​​

3030​​

3636​​


Calculate the probability:

44 outcomes out of 6×6=366\times6=36 options.


The probability that the two numbers multiply to give 1212 is 436=19\dfrac{4}{36}=\underline{\dfrac{1}{9}}.


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FAQs - Frequently Asked Questions

How do you represent sample space?

What is an experiment?

What is a sample space?

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