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# 9.8: Probability of Compound Events

Created by: CK-12

We begin this lesson with a reminder of probability.

The experimental probability is the ratio of the proposed outcome to the number of experiment trials.

$P(success)= \frac{number \ of \ times \ the \ event \ occured}{total \ number \ of \ trials \ of \ experiment}$

Probability can be expressed as a percentage, a fraction, a decimal, or a ratio.

This lesson will focus on compound events and the formulas used to determine the probability of such events.

Compound events are two simple events taken together, usually expressed as $A$ and $B$.

## Independent and Dependent Events

Example: Suppose you flip a coin and roll a die at the same time. What is the probability you will flip a head and roll a four?

These events are independent. Independent events occur when the outcome of one event does not affect the outcome of the second event. Rolling a four has no effect on tossing a head.

To find the probability of two independent events, multiply the probability of the first event by the probability of the second event.

$P(A \ and \ B)=P(A) \cdot P(B)$

Solution:

$P(tossing \ a \ head)&=\frac{1}{2}\\P(rolling \ a \ 4)&=\frac{1}{6}\\P(tossing \ a \ head \ AND \ rolling \ a \ 4)&=\frac{1}{2} \times \frac{1}{6}=\frac{1}{12}$

When events depend upon each other, they are called dependent events. Suppose you randomly draw a card from a standard deck then randomly draw a second card without replacing the first. The second probability is now different from the first.

To find the probability of two dependent events, multiply the probability of the first event by the probability of the second event, after the first event occurs.

$P(A \ and \ B)=P(A) \cdot P(B \ following \ A)$

Example: Two cards are chosen from a deck of cards. What is the probability that they both will be face cards?

Solution: Let $A = 1st \ Face \ card \ chosen$ and $B = 2nd \ Face \ card \ chosen$. The total number of face cards in the deck is $4 \times 3 = 12$.

$P(A)&= \frac{12}{52}\\P(B) & = \frac{11}{51}, \ \text{remember, one card has been removed.}$

$P(A \ AND \ B)= \frac{12}{52} \times \frac{11}{51} \ & or \ P(A \cap B) = \frac{12}{52} \times \frac{11}{51} =\frac{33}{663}\\P(A \cap B) & = \frac{11}{221}$

## Mutually Exclusive Events

Events that cannot happen at the same time are called mutually exclusive events. For example, a number cannot be both even and odd or you cannot have picked a single card from a deck of cards that is both a ten and a jack. Mutually inclusive events, however, can occur at the same time. For example a number can be both less than 5 and even or you can pick a card from a deck of cards that can be a club and a ten.

When finding the probability of events occurring at the same time, there is a concept known as the “double counting” feature. It happens when the intersection is counted twice.

In mutually exclusive events, $P(A \cap B)=\phi$, because they cannot happen at the same time.

To find the probability of either mutually exclusive events $A$ or $B$ occurring, use the following formula.

To find the probability of one or the other mutually exclusive or inclusive events, add the individual probabilities and subtract the probability they occur at the same time.

$P(A \ or \ B)=P(A)+P(B)-P(A \cap B)$

Example: Two cards are drawn from a deck of cards. Let:

$A$: $1^{st}$ card is a club

$B$: $1^{st}$ card is a 7

$C$: $2^{nd}$ card is a heart

Find the following probabilities:

(a) $P(A \ \text{or} \ B)$

(b) $P(B \ \text{or} \ A)$

(c) $P(A \ \text{and} \ C)$

Solution:

(a) $P(A \ or \ B)=\frac{13}{52}+\frac{4}{52}-\frac{1}{52}\!\\P(A \ or \ B) =\frac{16}{52}\!\\P(A \ or \ B) =\frac{4}{13}$

(b) $P(B \ or \ A)= \frac{4}{52} + \frac{13}{52}-\frac{1}{52}\!\\P(B \ or \ A) = \frac{16}{52}\!\\P(B \ or \ A) = \frac{4}{13}$

(c) $P(A \ and \ C) = \frac{13}{52} \times \frac{13}{52}\!\\P(A \ and \ C) = \frac{169}{2704}\!\\P(A \ and \ C) = \frac{1}{16}$

## Practice Set

1. Define independent events.

Are the following events independent or dependent?

1. Rolling a die and spinning a spinner
2. Choosing a book from the shelf then choosing another book without replacing the first
3. Tossing a coin six times then tossing it again
4. Choosing a card from a deck, replacing it, and choosing another card
5. If a die is tossed twice, what is the probability of rolling a 4 followed by a 5?
6. Define mutually exclusive.

Are these events mutually exclusive or mutually inclusive?

1. Rolling an even and an odd number on one die.
2. Rolling an even number and a multiple of three on one die.
3. Randomly drawing one card and the result is a jack and a heart.
4. Randomly drawing one card and the result is black and a diamond.
5. Choosing an orange and a fruit from the basket.
6. Choosing a vowel and a consonant from a Scrabble bag.
7. Two cards are drawn from a deck of cards. Determine the probability of each of the following events:
1. $P$(heart or club)
2. $P$(heart and club)
3. $P$(red or heart)
4. $P$(jack or heart)
5. $P$(red or ten)
6. $P$(red queen or black jack)
8. A box contains 5 purple and 8 yellow marbles. What is the probability of successfully drawing, in order, a purple marble and then a yellow marble? {Hint: In order means they are not replaced.}
9. A bag contains 4 yellow, 5 red, and 6 blue marbles. What is the probability of drawing, in order, 2 red, 1 blue, and 2 yellow marbles?
10. A card is chosen at random. What is the probability that the card is black and is a 7?

Mixed Review

1. A circle is inscribed within a square, meaning the circle's diameter is equal to the square’s side length. The length of the square is 16 centimeters. Suppose you randomly threw a dart at the figure. What is the probability the dart will land in the square, but not in the circle?
2. Why is $7-14x^4+7xy^5-1x^{-1}=8x^2 y^3$ not considered a polynomial?
3. Factor $72b^5 m^3 w^9-6(bm)^2 w^6$.
4. Simplify $2^5-7^3 a^3 b^7+3^5 a^3 b^7-2^3$.
5. Bleach breaks down cotton at a rate of 0.125% with each application. A shirt is 100% cotton.
1. Write the equation to represent the percentage of cotton remaining after $w$ washes.
2. What percentage remains after 11 washes?
3. After how many washes will 75% be remaining?
6. Evaluate $\frac{(100 \div 4 \times 2-49)^2}{9-2 \times 3+2^2}$.

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Feb 22, 2012

Dec 11, 2014