Theoretical probability

Theoretical probability is what we expect in an ideal, fair situation, where every outcome is equally likely. No experiment is needed: we reason it out.

P(event) = number of favourable outcomes ÷ number of possible outcomes. ('Favourable' just means the outcomes you are looking for.)

Example 3. A 4 on a fair die: one favourable outcome out of six, so P(4) = 1/6 ≈ 0.167, or 16.7%.

Example 4. A letter picked from PROBABILITY. It has 11 letters, and 2 of them are B, so P(B) = 2/11 ≈ 0.182.

Count repeats separately. In PEACE, E appears twice, so P(E) = 2/5.

Everything adds to 1. The probabilities of all the outcomes add up to 1, so P(not A) = 1 – P(A).

The formula only works when the outcomes really are equally likely. A fair coin or a fair die: yes. A paper cup, or a car that 'starts or doesn't': no.