‘The workings of probability’ defines a number of axioms which can be applied when calculating probability. The Addition Law states that when two events are mutually exclusive, the probability that one occurs in the sum of their individual probabilities. To assess the probability of both of two events occurring, the Multiplication Law states that you should multiply the probability of the first event by the probability of the second, conditional on the first even occurring. For more than two events, apply the Multiplication Law for each stage, then multiply the probabilities that each stage occurs together. Incorrectly labelling events as independent, disjoint, or conditional are major sources of error.