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Probability

Probability is a measure of how likely something is to happen. It is expressed with a number between 0 and 1; 0 means "impossible" and 1 means "certain".

For example: suppose you are asked to randomly choose a letter of the alphabet between J and N, inclusive. The possibilities are J, K, L, M, N. The probability that you pick a vowel is 0. The probability that you pick a consonant is 1. The probability that you pick K is 1/5 = 0.2.

In general, the probability that an event occurs is given by:

There are six possible outcomes: 1, 2, 3, 4, 5 and 6. There are two favorable outcomes: 5 and 6. So the probability is:

This is a theoretical probability, as opposed to experimental probability, which is the observed number of favorable outcomes out of a certain number of trials. For instance, suppose you rolled the six-sided die 5 times, and got the following results: 2, 6, 4, 5, 6. Then the experimental probability of rolling a number greater than four would be 3/6 or one-half.

As the number of trials increases, the experimental probability will become close to the theoretical probability.