Past exam of the year 1 of the computer science course of the University of Oxford / 2016 / Discrete mathematics, continuous mathematics and probability / 8 / a / i / Solution

A partition of a set $\Omega$ is a collection of nonempty subsets $B_i$ such that $B_i\cap B_j=\varnothing$ for $i\ne j$ and ${\bigcup }_{i\in I}{B}_i=\Omega$. Thus every outcome lies in exactly one cell. For a probability-space partition the cells must also be measurable events. Nonempty cells may nevertheless have probability zero.