An event is a collection of possible outcomes.
For example, the act of dolling a dice is an event. The collection of possible outcomes is that you will roll a 1, 2, 3, 4, 5 or 6. When you consider the event of rolling a 6, there are two possible events: you roll a 6 or you do not roll a dice. There is one possible way of rolling a 6, while there are 5 possible ways of not rolling a 6 because you can roll a 1, 2, 3, 4, or 5.
We define the probability of an event, E, to be:
P(E) = (number of events within E)/ (total number of possible outcomes)
Let’s look at an some examples:
Example One: When you roll a die, what is the probability of rolling a 6?
The event we are looking at is rolling a 6 when you roll a die.
There is one possible way to roll a 6 on a die.
There are 6 possible outcomes when you roll a die; rolling a 1, 2, 3, 4, 5, or 6.
P(rolling a 6) = 1/6
Therefore, the probability of rolling a 6 when you roll a die is 1/6.
Example Two: You have 4 cards, each with one of the numbers, 5, 6, 7, or 8, written on them. You pick a card at random. What is the probability that the card you pick will have a 6 written on it?
The event we are looking at is picking the card labelled with a 6.
There is one possible way to pick the card labelled with a 6.
There are 4 possible outcomes when you choose one of the four cards at random.
P(choosing the 6 card) = 1/4
Therefore, the probability of choosing the card labelled with a 6 is 1/4.
Example Three: Lamont owns 14 T-shirts, 2 of which are red. If Lamont randomly selects a T-shirt to wear, what is the probability it will be red?
The event we are looking at is Lamont randomly selecting a red T-shirt.
There are two possible ways of Lamont selecting a red T-shirt.
There are 14 possible ways of Lamont selecting a T-shirt to wear.
P(Lamont selecting a red shirt) = 2/14 = 1/7
Therefore, the probability of Lamont choosing a red T-shirt to wear is 1/7.
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This article was written for you by Mia, one of the tutors with Test Prep Academy.