Past exam of the mathematics course of the University of Oxford / 2015 / Prelims / Mathematics III / 4 / a / i / Solution

The events $B_1,B_2,\dots$ form a partition of a sample space $\Omega$ when they are pairwise disjoint sets and cover the whole sample space:Thus exactly one of these events occurs on every outcome. Under the usual set partition convention the parts are nonempty; empty parts can simply be discarded.