The rules of probability are useful for expressing the likelihood of a certain event occuring. To calculate the probability of an event, one would find the number of times a certain event occurs and then divide that quantity by the number of all possible outcomes. For example, if a person were to buy five lottery tickets and there are 100 million tickets in circulation then the odds of that person having the winning ticket would be 1/20,000,000.
The probability of two or more independent events occuring together is found by multiplying their respective probabilities.
To illustrate this principle, here are a few examples:
coins are flipped. What is the probability of getting all heads?
Number of flips:
| Number of Coins | Number of Trials | Heads | Tails |
|---|---|---|---|
| 2 | 0 | 0 | 0 |
| Number of times getting all heads | Number of times getting all tails |
|---|---|
| 0 | 0 |
dice are rolled. What is the probability of getting a one on the first die and an odd number on the second die?
Number of rolls:
| ones | twos | threes | fours | fives | sixes |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 | 0 |
The probability of any one of several different outcomes occuring is found by adding the probabilities of these events.