Probability Rules

Question 1

Rule #1: Create a Fraction

Answer 1

To find a probability, use this formula, which creates a fraction you can use to solve the problem:

P = Number of possible desired outcomes

Number of total possible outcomes

Question 2

Rule #2: Multiply Consecutive Probabilities

Answer 2

A consecutive probability is the odds that a certain event will occur a specific number of times in a row; for example, if you flip a coin twice, the probability that it comes up heads both times. To solve a consecutive probability problem, find the probability of each event seperately and then multiply them together.

Example:

What’s the probability of tossing a die twice and getting a 5 on the first toss and a 6 on the second toss? Treat each toss seperately.

The probability of getting a 5 is 1/6. The probability of getting a 6 is 1/6.

Multiply consecutive probabilities: 1/6 x 1/6 = 1/36

*The probability is the same whether you toss one die twice or two dice at the same time.

Question 3

Rule #3: Add Either/Or Probabilities on a Single Event

Answer 3

An either/or probability on a single event is one in which either of two outcomes may occur; for example, the probability that you reach into a bag containing 10 blue, 10 red, and 10 green marbles and pull out a blue or a red marble. To solve an either/or probability problem, find the probability of each event seperately and then add them together.

Example:

What’s the probability of tossing a die once and getting either a 5 or a 6?

Either outcome has a one in six chance of occurring:

1/6 + 1/6 = 2/6 = 1/3

The probability of rolling either a 5 or a 6 on a single die roll is 1/3.