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Probability Of An Event

In these lessons, we will learn how to find the probability of an event.

Related Topics:
Probability of Mutually Exclusive Events ("Or" events),
Probability of Independent Events ("And" events),
Probability of Dependent Events ("And" events without replacement),
Other Lessons on Probability

In an experiment, an event is the result that we are interested in.

The probability of an event A, written P(A), is defined as


When a fair dice is thrown, what is the probability of getting

a) the number 5
b) a number that is a multiple of 3
c) a number that is greater than 6
d) a number that is less than 7


A fair die is an unbiased die where each of the six numbers is equally likely to turn up.

S = {1, 2, 3, 4, 5, 6}

a) Let A = event of getting the number 5 = {5}

Let n(A) = number of outcomes in event A = 1

n(S) = number of outcomes in S = 6

b) Let B = event of getting a multiple of 3

Multiple of 3 = {3, 6}

c) Let C = event of getting a number greater than 6

There is no number greater than 6 in the sample space S.

C ={}

A probability of 0 means the event will never occur.

d) Let D = event of getting a number less than 7

Numbers less than 7 = {1, 2, 3, 4, 5, 6}

A probability of 1 means the event will always occur.

Example :

Each of the letters HELLO is written on a card. A card is chosen at random from the bag. What is the probability of getting the letter ‘L’?


Since the card is randomly selected, it means that each card has the same chance of being selected.

S = {H, E, L 1, L 2, O} There are two cards with the letter ‘L’

Let A = event of getting the letter ‘L’ = {L 1, L 2}

How to calculate the Probability of Simple Events
Example 1: Find the probability of the next person you meeting having a phone number that ends with 5?
Example 2: Find the probability of getting all heads if you flip 3 coins?
Example 3: Find the probability that the person that meet next has a birthday in February? (Non-leap year) How to determine the probability of single events and express it as a ratio?
Example 1: What s the probability of drawing a spade from a deck of 52 playing cards?
Example 2: If a dart randomly hits the board below, what is the probability that it will hit the shaded region?
Example 3: It is Cindy's turn to spin in a game that she is playing with her friends. What is the probability that Cindy will have to move back on this spin? How to find the probability of an event?

How to find the probability of an event?
Example: Suppose you spin the spinner below. Find the probability of spinning 4. Write the probability as a fraction, a decimal and a percent.

Rotate to landscape screen format on a mobile phone or small tablet to use the Mathway widget, a free math problem solver that answers your questions with step-by-step explanations.

You can use the free Mathway calculator and problem solver below to practice Algebra or other math topics. Try the given examples, or type in your own problem and check your answer with the step-by-step explanations.

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