Probability is measured using numbers from 0 to 1. A probability that's close to 0 means something is unlikely to happen, such as correctly guessing a stranger's birthday. In fact, you're much more likely to guess a stranger's birthday
incorrectly.
The probability of guessing the birthday incorrectly is close to 1.
When determining probabilities, the possible results of an action are called
outcomes.
For example, if you roll a six-sided die, there are six possible outcomes. The die can land on 1, 2, 3, 4, 5, or 6.
Anevent is a set of those outcomes. If you roll a six-sided die, the outcomes of rolling a 4, 5, or 6 make up the event of rolling a number greater than 3.
The probability of an event happening, written as P(event), is the number of favorableoutcomes divided by the total possible outcomes. Favorable outcomes are the outcomes that make up the event. You can write P(event) as a fraction:
P(event)
=
number of favorable outcomes
number of possible outcomes
For example, the probability of rolling a six-sided die and having it land on 1 can be written as P(1).
There are 6 possible outcomes: 1, 2, 3, 4, 5, or 6.
There is 1 favorable outcome: 1.
So,
P(1)
=
1
6
.
The probability of rolling a six-sided die and having it land on a number greater than 3 can be written as P(>3).
There are 6 possible outcomes: 1, 2, 3, 4, 5, or 6.
There are 3 favorable outcomes: 4, 5, and 6.
So,
P(>3)
=
3
6
=
1
2
.
Probabilities can also be written as percentages. Since
