Probability tells how likely something is: 0 means impossible and 1 means certain. An outcome is one possible result. For a fair coin, heads and tails have the same chance. P(heads) means “the probability of heads,” which is 1/2. When all outcomes are equally likely, divide the number that fit the question (favorable outcomes) by the total number of outcomes.
The sample space lists all possible outcomes. A coin’s sample space is heads and tails. For a die labeled 1 to 6, it is 1, 2, 3, 4, 5, 6. The complement of an event includes all outcomes outside that event. For “red,” the complement is “not red.” Their probabilities total 1, so P(not red) = 1 − P(red).
Theoretical probability uses a mathematical model. For a fair coin, two equally likely sides give a heads probability of 1/2. Experimental probability uses actual results: if 7 of 20 flips are heads, it is 7/20. Each flip is one trial, or try. These probabilities need not be equal. Events are independent when knowing one result does not change the chance of the other. For independent events, multiply to find the probability that both happen: P(A and B) = P(A) × P(B).
