4. Basic Probability

Probability measures how likely something is, from 0 (impossible) to 1 (certain). For equally likely outcomes:
\[ P(\text{event}) = \frac{\text{favorable outcomes}}{\text{total outcomes}}. \]

Worked example: a bag has 3 red, 2 blue, and 5 green marbles (10 total). The probability of drawing red is
\[ P(\text{red}) = \frac{3}{10} = 0.3 = 30\%. \]

Worked example (a number cube): the probability of rolling a number greater than 4 (that is, 5 or 6) is
\[ P = \frac{2}{6} = \frac{1}{3}. \]

The probability that an event does not happen is \(1 - P(\text{event})\). If rain is 70% likely, then no rain is \(1 - 0.70 = 0.30\), or 30%.