Probability Rules

Probability is a crucial concept in mathematics and statistics that enables us to quantify uncertainty and make informed decisions in various fields. Whether it’s predicting the outcome of an experiment or assessing risk in finance, probability rules provide a systematic way to analyze and understand random phenomena.

In this article, we’ll explore the fundamental principles of probability, starting with its definition and then delving into the essential rules governing its calculations.

What is Probability?

Probability is the measure of the likelihood of an event occurring. It is represented as a number between 0 and 1, where 0 indicates impossibility (the event will not occur) and 1 indicates certainty (the event will occur).

In practical terms, probability allows us to quantify uncertainty and assess the relative chances of different outcomes.

Probability Rules

Various probability rules used to solve problems are:

Probability Rules

Addition Rule

Addition rule of probability deals with the probability of the union of two events. If A and B are two events, then the probability of either event A or event B occurring is given by:

P(A∪B) = P(A) + P(B) – P(A∩B)

where,

• P(A∩B) represents the probability of both events A and B occurring simultaneously

This rule applies when events A and B are not mutually exclusive.

Example: Suppose we toss a fair coin. Let A be the event of getting a head, and B be the event of getting a tail. The probability of either getting a head or a tail is given by:

P(AUB) = P(A) + P(B)

= 1/2 + 1/2 = 1

Multiplication Rule

Multiplication rule of probability applies when we want to find the probability of the intersection of two independent events. If A and B are independent events, then the probability of both events A and B occurring is given by:

P(A∩B) = P(A) × P(B)

This rule holds true when the occurrence of one event does not affect the probability of the other event.

Example: Consider rolling a fair six-sided die. Let A be the event of rolling an even number, and B be the event of rolling a number less than 4. Since these events are independent, the probability of rolling an even number and a number less than 4 is given by:

P(A∩B) = P(A) × P(B)

P(A) = 3/6 = 1/2

P(B) = 3/6 = 1/2

P(A∩B) = 1/2×1/2 = 1/4

Complement Rule

Complement rule deals with the probability of the complement of an event. If A is an event, then the probability of the complement of A (i.e., the event not occurring) is given by:

P(A′) = 1 – P(A)

Example: Suppose we draw a card from a standard deck of 52 playing cards. Let A be the event of drawing a heart. The probability of not drawing a heart (drawing a card that is not a heart) is given by:

P(A) = 13/52 = 1/4

P(A’) = 1 – P(A)

P(A’) = 1 – 1/4 = 3/4

Conditional Probability

Conditional probability is the probability of an event occurring given that another event has already occurred. If A and B are events, then the conditional probability of A given B is denoted by P(A∣B) and calculated as:

P(A∣B) = P(A∩B)/P(B)

This rule quantifies how the probability of one event changes in light of the occurrence of another event.

Example: Let’s continue with the example of drawing a card from a standard deck of 52 playing cards. Suppose B is the event of drawing a face card (jack, queen, or king), and A is the event of drawing a heart. The probability of drawing a heart given that the card drawn is a face card is given by:

P(A) = 13/52 = 1/4

P(B) = 12/52

P(A∩B) = 3/52

P(A∣B) = P(A∩B)/P(B)

P(A∣B) = (3/52)/(12/52) = 1/4

This means that given the card drawn is a face card, there is a 1/4 chance that it is also a heart.

Summary

Probability rules provide a systematic framework for quantifying uncertainty and analyzing random phenomena. By understanding these rules and applying them in various scenarios, we can make informed decisions and predictions in fields ranging from science and engineering to finance and everyday life.

Learn More:

• Chance and Probability
• Empirical Probability
• Probability Theory
• Addition Rule for Probability
• Practice Problems on Probability

Sample Questions on Probability Rules

Question 1: You flip a fair coin twice. What is the probability of getting exactly one head?

Solution:

To solve this question, we can use the complement rule.

Probability of getting exactly one head is the complement of getting either two heads or two tails.

Probability of getting two heads: [Tex]P(\text{HH}) = \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}[/Tex]

Probability of getting two tails: [Tex]P(\text{TT}) = \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}[/Tex]

So, the probability of getting exactly one head is:

[Tex]1 – P(\text{HH}) – P(\text{TT}) = 1 – \frac{1}{4} – \frac{1}{4} = \frac{1}{2}[/Tex]

Question 2: A bag contains 5 red balls and 3 blue balls. You randomly select two balls from the bag without replacement. What is the probability of selecting one red ball and one blue ball?

Solution:

We can solve this question using the multiplication rule.

First, we find the probability of selecting a red ball, then the probability of selecting a blue ball after selecting a red ball.

[Tex]P(\text{red}) = \frac{5}{8}[/Tex]

[Tex]P(\text{blue after red}) = \frac{3}{7}[/Tex]

So, the probability of selecting one red ball and one blue ball is:

[Tex]P(\text{red and blue}) = P(\text{red}) \times P(\text{blue after red}) = \frac{5}{8} \times \frac{3}{7} = \frac{15}{56}[/Tex]

Question 3: You roll a fair six-sided die. What is the probability of rolling a number divisible by 2 or 3?

Solution:

We can solve this question using the addition rule.

We find the probability of rolling a number divisible by 2, the probability of rolling a number divisible by 3, and subtract the probability of rolling a number divisible by both 2 and 3 (i.e., 6).

Probability of rolling a number divisible by 2: [Tex]P(\text{divisible by 2}) = \frac{3}{6} = \frac{1}{2}[/Tex]

Probability of rolling a number divisible by 3: [Tex]P(\text{divisible by 3}) = \frac{2}{6} = \frac{1}{3}[/Tex]

Probability of rolling a number divisible by both 2 and 3: [Tex]P(\text{divisible by 6}) = \frac{1}{6}[/Tex]

So, the probability of rolling a number divisible by 2 or 3 is:

[Tex]P(\text{divisible by 2 or 3}) = P(\text{divisible by 2}) + P(\text{divisible by 3}) – P(\text{divisible by 6}) = \frac{1}{2} + \frac{1}{3} – \frac{1}{6} = \frac{5}{6}[/Tex]

Question 4: A card is drawn from a standard deck of 52 playing cards. What is the probability of drawing a face card or a heart?

Solution:

We can solve this question using the addition rule.

We find the probability of drawing a face card, the probability of drawing a heart, and subtract the probability of drawing a card that is both a face card and a heart.

Probability of drawing a face card: [Tex]P(\text{face card}) = \frac{12}{52}[/Tex] (There are 3 face cards in each suit)

Probability of drawing a heart: [Tex]P(\text{heart}) = \frac{13}{52}[/Tex]

Probability of drawing a card that is both a face card and a heart: [Tex]P(\text{face card and heart}) = \frac{3}{52}[/Tex] (There are 3 face cards that are hearts)

So, the probability of drawing a face card or a heart is:

[Tex]P(\text{face card or heart}) = P(\text{face card}) + P(\text{heart}) – P(\text{face card and heart}) = \frac{12}{52} + \frac{13}{52} – \frac{3}{52} = \frac{22}{52} = \frac{11}{26}[/Tex]

Question 5: Two dice are rolled. What is the probability of getting a sum greater than 9?

Solution:

We can solve this question by listing all possible outcomes of rolling two dice and counting the outcomes where the sum of the numbers is greater than 9.

There are 6×6 = 36 possible outcomes when rolling two dice. Outcomes with a sum greater than 9:

(4, 6), (5, 5), (5, 6), (6, 4), (6, 5), (6, 6)

So, there are 6 outcomes where the sum is greater than 9.Therefore, the probability of getting a sum greater than 9 is:

[Tex]P(\text{sum > 9}) = \frac{6}{36} = \frac{1}{6}[/Tex]

FAQs on Probability Rules

What is difference between independent and mutually exclusive events in probability?

Independent events are events where the occurrence of one event does not affect the probability of the other event. Mutually exclusive events, on the other hand, are events that cannot occur simultaneously. For example, when rolling a six-sided die, the outcomes of getting an odd number and getting a number greater than ‘4’ are independent, while the outcomes of getting a ‘2’ and getting a ‘3’ are mutually exclusive.

How to calculate probability of compound events involving AND or OR operations?

Probability of compound events involving AND operations is calculated using the multiplication rule, while the probability of compound events involving OR operations is calculated using the addition rule. For example, to find the probability of rolling a number less than 3 or an even number on a six-sided die, you would use the addition rule.

What are mutually exclusive events with an example?

Mutually exclusive events are events that cannot occur simultaneously. For example, when rolling a six-sided die, the outcomes of getting a 2 and getting a 3 are mutually exclusive because it is not possible to roll both a 2 and a 3 on the same die. In general, if two events A and B are mutually exclusive, then the probability of both events occurring (P(A∩B)) is equal to 0.

How to use complement rule in probability calculations?

Complement rule states that the probability of the complement of an event (the event not occurring) is 1 minus the probability of the event occurring. It is represented as: P(A′) = 1 − P(A) where A′ represents the complement of event A.