Probability
Probability is a numerical measure of how likely an event is to happen.
Probability is measured in fractions between 0 and 1. (0 is impossible; 1 is
certain.) Sometimes, probability is represented as a percentage -- from 0 percent to 100 percent.
In probability, we talk about events and outcomes. An event is anything that happens. An outcome is the result
of what happens. The following are examples of events...
1) A coin flip.
2) Rolling a die.
3) Drawing a card from a deck of cards.
The following are possible outcomes of events…
1) A coin toss has two possible outcomes. The outcomes are "heads" and "tails".
2) Rolling a regular six-sided die has six possible outcomes. You may get a side with 1, 2, 3, 4, 5, or 6 dots.
The probability of an outcome for a particular event is a number telling us how likely a particular outcome is to
occur. You can determine the probability of a particular outcome by dividing the number of times that the
outcome has occurred by the total number of events.
Example:
To find the probability that a flipped coin will come up heads, for example, you might flip the
coin 25 times. If the coin turns up heads 10 times, then the probability that the coin will land heads up on the
next flip is
10
2
=
25
5
Example:
Suppose there are 10 balls in a bucket numbered as follows: 1, 1, 2, 3, 4, 4, 4, 5, 6, and 6. A
single ball is randomly chosen from the bucket. What is the probability of drawing a ball numbered 1? There are
2 ways to draw a 1, since there are two balls numbered 1. The total possible number of outcomes is 10, since
there are 10 balls. The probability of drawing a 1 is the ratio
2
1
=
10
5
Example:
Suppose a regular die is rolled. What is the probability of getting a 3 or a 6? There are a total of
6 possible outcomes. Rolling a 3 or a 6 are two of them, so the probability is the ratio of 2/6 = 1/3.
2
1
=
6
3