Probability describes how likely an event is when the result is random. It is always a number from 0 to 1, where 0 means impossible and 1 means certain.

An outcome is one possible result, such as landing on blue. An event is the result you care about, and we write its probability as $P(blue)$.

If a question lists every possible outcome, their probabilities must add to 1. In symbols, $P(all\; outcomes)=1$, and this helps you find missing values and check your answers.