Equally likely events are events that have the same theoretical probability (or likelihood) of occurring. Getting a 1, 2 or 3 on the toss of a die and getting a 4, 5 or 6 on the toss of a die are equally likely events, since the probabilities of each event are equal.

The size of the sample space is the total number of possible outcomes. For example, when you roll 1 die, the sample space is 1, 2, 3, 4, 5, or 6. So the size of the sample space is 6.

If outcomes are equally likely, then the probability of an event occurring is the number in the event divided by the number in the sample space. The probability of rolling a six on a single roll of a die is 1/6 because there is only 1 way to roll a six out of 6 ways it could be rolled.

Use the specific multiplication rule formula. Just multiply the probability of the first event by the second. For example, if the probability of event A is 2/9 and the probability of event B is 3/9 then the probability of both events happening at the same time is (2/9)*(3/9) = 6/81 = 2/27.

In the case of three events, A, B, and C, the probability of the intersection P(A and B and C) = P(A)P(B|A)P(C|A and B).

Hence, the probability of getting a multiple of 3 when a die is thrown is 1/3.

Probabilities can be represented as a ratio, percentage, fraction or as a decimal; I often point this out to students, so they are alert to the multiple ways we represent odds. This often brings up difficulties and fears with basic mathematics including fractions, percentages and ratios.

A risk event that is certain not to occur has, by definition, probability equal to zero….Other Risk Management Probability Definitions.

From Longman Dictionary of Contemporary English an even chancea situation in which it is just as likely that something will happen as not happen I think we have an even chance of winning.

Probability is the ratio of the times an event is likely to occur divided by the total possible events. In the case of our die, there are six possible events, and there is one likely event for each number with each roll, or 1/6.

You use probability in daily life to make decisions when you don’t know for sure what the outcome will be. Most of the time, you won’t perform actual probability problems, but you’ll use subjective probability to make judgment calls and determine the best course of action.

To find the probability of at least one of something, calculate the probability of none and then subtract that result from 1. That is, P(at least one) = 1 – P(none).