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# Mutually Exclusive Events

## Probability of two events that cannot occur at the same time P(A or B) = P(A) + P(B)

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Union of Compound Events

#### Objective

In this lesson, you will learn about calculating the probability that any one of multiple mutually exclusive independent events will occur in a single experiment.

#### Concept

If you think it through, it should make sense that the probability of pulling one Queen at random from a standard deck is  $\frac{4}{52}$ or $\frac{1}{13}$ , since there are 4 Queens in a standard 52 card deck. How then would you calculate the probability of pulling a Queen OR a King from the same deck?

After this lesson on the union of compound events , we’ll return to this question and work out the answer.

#### Guidance

When multiple independent events may occur during a particular experiment, there are a couple of different types of outcomes you may need to consider:

• Intersection: the probability of both or all of the events you are calculating happening at the same time (less likely).
• Union: the probability of any one of multiple events happening at a given time (more likely).

In this lesson, we will focus on union . Calculating the union is relatively easy, you just add up the individual probabilities of the events:

$P(x \ or \ y)=P(x)+P(y)$

This can also be thought of as:

$P(x \ or \ y)=\frac{(\text{number of outcomes where} \ x \ \text{is true}) +(\text{number of outcomes where} \ y \ \text{is true})}{\text{total number of possible outcomes}}$

It is really just that simple! It is intuitive also, assuming there is no overlap (which we will consider later), it just makes sense to think that if you have a 20% probability of one thing happening, and a 30% probability of another, then you have a 50% probability of one of the two of them happening during a given experiment.

Example A

You are given a big containing 15 equally sized marbles. You know there are 5 yellow marbles, 5 blue marbles, and 5 green marbles in the bag. What is the statistical probability that you would pull a yellow or green marble out, if you reach in the bag and grab a marble at random?

Solution: Recall the formula for the union of simple probabilities:

$P(x \ or \ y)=\frac{(\text{number of outcomes where} \ x \ \text{is true}) +(\text{number of outcomes where} \ y \ \text{is true})}{\text{total number of possible outcomes}}$

In this case, we have:

$P(yellow \ or \ green)=\frac{5 \ yellow \ marbles+5 \ green \ marbles}{15 \ total \ marbles}$

Which would reduce to:

$P(yellow \ or \ green)=\frac{2}{3} \ or \ 66.6 \bar{6} \%$

Example B

What is the probability of rolling an odd or even number on a standard six-sided die?

Solution: A standard die has three odd numbers (1, 3, 5) and three even numbers (2, 4, 6). Therefore, the probability of rolling an odd or even number is:

$P(odd \ or \ even)=\frac{3 \ odd + 3 \ even}{6 \ total}=\frac{6}{6}$

Reducing to:

$P(odd \ or \ even)=1 \ or \ 100 \%$

Example C

If Lawrence is playing with a standard 52-card deck, what is the probability of pulling a 2, a 4, or a 6 out of the deck at random?

Solution: Let’s solve this one as the total of the individual probabilities. Lawrence’s probability of pulling a 2, 4, or 6 is the same as the union of the probability of each possible outcome:

$P(2, 4, \ or \ 6)=P(2)+P(4)+P(6)=\frac{1}{13}+\frac{1}{13}+\frac{1}{13}=\frac{3}{13} \ or \ 23.1 \%$

Concept Problem Revisited

It should make sense now that the probability of pulling one Queen at random from a standard deck is  $\frac{4}{52}$ or $\frac{1}{13}$ , since there are 4 Queens in a standard 52 card deck. How then would you calculate the probability of pulling a Queen OR a King from the same deck?

Remember that the union of multiple probabilities is simple the total sum of all of the individual probabilities:

$P(Queen \ or \ King)=\frac{4 \ Queens+4 \ Kings}{52 \ Cards}=\frac{8}{52} \ or \ \frac{4}{26} \ or \ \frac{2}{13}=15.4 \%$

#### Vocabulary

A statistical probability is the calculated probability that a given outcome will occur if the same experiment were completed an infinite number of times.

The probability of the union of multiple, mutually exclusive, events is the sum of the probabilities of each of the individual outcomes occurring during a given trial.

An event is any collection of the outcomes of an experiment.

Mutually exclusive events cannot occur at the same time (they have no overlap). For instance, a single coin flip cannot be both heads and tails.

An independent event is an event that is unaffected by any other event occurring before or after it.

An outcome is the result of a single trial.

#### Guided Practice

1. What is the statistical probability of pulling either the only red or the only blue marble out of a bag with 12 marbles in it?
2. What is the probability of a spinner landing on “2”, “3”, or “6” if there are 6 equally spaced points on the spinner?
3. What is the probability of pulling a red or black card at random from a standard deck?
4. What probability of picking a red or green marble from a bag with 5 red, 7 green, 6 blue, and 14 yellow marbles in it?
5. What is the of shaking the hand of a student wearing red if you randomly shake the hand of one person in a room containing the following mix of students?
• 13 female students wearing blue
• 7 male students wearing blue
• 6 female students wearing red
• 9 males students wearing red
• 18 female students wearing green
• 21 male students wearing green

Solutions:

1. $P(red \ or \ blue)=\frac{1\text{ red marble} + 1\text{ blue marble}}{12\text{ total marbles}}=\frac{2}{12} \ or \ \frac{1}{6} \ or \ 16.6 \%$
2. $P(2 \ or \ 3 \ or \ 6)=\frac{1 \text{ number }6 + 1\text{ number }2 + 1\text{ number }3}{6\text{ total numbers}}=\frac{3}{6} \ or \ \frac{1}{2} \ or \ 50 \%$
3. $P(red \ or \ black)=\frac{26\text{ red cards} + 26\text{ black cards}}{52\text{ total cards}}=\frac{52}{52}=\frac{1}{1} \ or \ 100 \%$
4. $P(red \ or \ green)=\frac{5\text{ red marbles}+7\text{ green marbles}}{32\text{ total marbles}}=\frac{12}{32} \ or \ \frac{6}{16} \ or \ \frac{3}{8} \ or \ 37.5 \%$
5. $P(red)=\frac{6\text{ females wearing red}+9\text{ males wearing red}}{74\text{ total students}}=\frac{15}{74} \ or \ 20.3 \%$

#### Practice

1. What is the probability of rolling a standard die and getting between a 1 and 6 (inclusive)?
2. What is the probability of pulling one card from a standard deck and it being an 8, a 3, or a queen?
3. What is the probability of rolling a 5 or a 2 on an 8-sided die?
4. What is the probability of pulling one card from a standard deck and it being a spade, a diamond, or a club?
5. What is the probability of rolling a 1, 3, or 5 on a 7-sided die?
6. What is the probability of pulling one card from a standard deck and it being a king, a 4, or a 8?
7. What is the probability of pulling a yellow or blue candy from a bag containing 35 candies equally distributed among yellow, blue, green, red, and brown candies?
8. What is the probability of spinning 2, 4, or 7 on a 10-space spinner (equally spaced)?
9. What is the probability of rolling a 1, 3, 5, or 6 on a 20-sided die?
10. A car factory creates cars in the following ratio: 3 green, 2 blue, 7 white, 2 black and 1 brown. What is the probability that a randomly selected car will be either blue or brown?
11. There are 4 flavors of donuts on the shelf: glazed, sprinkles, plain, and powdered sugar. If there are equal numbers of each of the non-plain donuts, and half as many plain as any one of the others, what is the probability of randomly choosing a plain donut out of all donuts on the shelf?
12. What is the probability of randomly choosing a red Ace or a black King from a standard deck?
13. What is the probability of rolling a prime number or an even number on a standard die?
14. Mr. Spence’s class has 13 students. 4 students are wearing coats, 3 are wearing vests, 3 are wearing hoodies, and the rest are in t-shirts. What is the probability that Mr. Spence will randomly call the name of a student wearing a coat or a vest?
15. In the same class, what is the probability that Mr. Spence will randomly call the name of a student in a hoodie or t-shirt?

### Vocabulary Language: English

Event

Event

An event is a set of one or more possible results of a probability experiment.
Independent Events

Independent Events

Two events are independent if the occurrence of one event does not impact the probability of the other event.
Intersection

Intersection

Intersection is the probability of both or all of the events you are calculating happening at the same time (less likely).
Mutually Exclusive Events

Mutually Exclusive Events

Mutually exclusive events have no common outcomes.
Outcome

Outcome

An outcome of a probability experiment is one possible end result.
Permutation

Permutation

A permutation is an arrangement of objects where order is important.
statistical probability

statistical probability

A statistical probability is the calculated probability that a given outcome will occur if the same experiment were completed an infinite number of times.
union

union

$\cup$ is a symbol that stands for union and is used to connect two groups together. It is associated with the logical term OR.