Let $\mathcal{P}_{1}$ and $\mathcal{P}_{2}$ be finite partitions of $\Omega$. Show that the coarsest partition (i.e. with least number of sets) which refines them both consists of all intersections $A \cap B$, where $A \in \mathcal{P}_{1}, B \in \mathcal{P}_{2}$.