Theoretical probability is calculated from equally likely outcomes using the formula:
$$P(\text{event}) = \frac{\text{number of favourable outcomes}}{\text{total number of equally likely outcomes}}$$
Example: A fair dice is rolled. The probability of rolling a prime number (2, 3, 5): $$P(\text{prime}) = \frac{3}{6} = \frac{1}{2}$$
Theoretical probability assumes all outcomes are equally likely — this is different from experimental (relative frequency) probability.
Probabilities are always between 0 and 1 (or 0% and 100%). A probability of 0 means impossible; 1 means certain.
Combined events: use sample space diagrams or probability trees to list all outcomes systematically.
Example: Two fair coins tossed: sample space = {HH, HT, TH, TT} $$P(\text{exactly one head}) = \frac{2}{4} = \frac{1}{2}$$
Common error: not listing all possible outcomes, or not simplifying the fraction.