Let $\mu^* : \mathcal{P}(X) \rightarrow [0, \infty]$ be an outer measure, and let $M$ denote the set of $\mu^*$-measurable sets.
Let $A \subseteq X$ and let $E,F \in M$.
Why is the following statement true?
$\mu^*(A \cap E^c) = \mu^*(A \cap F \cap E^c) + \mu^*(A \cap E^c \cap F^c)$
EDIT: Is it because $F \in M$, with $(A \cap E^c)$ serving as our "test set"?