From a well-shuffled pack idea: a box has cards numbered 1 to 20. What is the probability of drawing a multiple of 5? (As a fraction.)

If all outcomes are equally likely, the probability of an event is:

P(event) = (number of outcomes favouring the event) / (total possible outcomes).

Examples on a fair die:
- P(rolling a 4) = $\frac{1}{6}$.
- P(rolling an even number) = $\frac{3}{6}=\frac{1}{2}$ (favourable: 2, 4, 6).
- P(rolling a number > 6) = $\frac{0}{6}=0$ (impossible).
- P(rolling a number ≤ 6) = $\frac{6}{6}=1$ (certain).

Probability is always between 0 (impossible) and 1 (certain). The probabilities of all possible outcomes sum to 1.