Past exam of the integration course of the University of Oxford / 2017 / 1 / a / i

If ${\sum }_n{m}^{\ast }(A_n)=\infty$, the required inequality is automatic. Otherwise each $A_n$ has finite Lebesgue outer measure. Fix $\epsilon >0$. By the defining infimum, for each $n\ge 1$ choose a countable interval cover $\{J_{n,k}:k\ge 1\}$ withThe family indexed by $(n,k)\in {\mathbb{N}}^2$ is countable and covers ${\bigcup }_n{A}_n$. Since all lengths are nonnegative, its total length is at most ${\sum }_n{m}^{\ast }(A_n)+\epsilon$. Taking the infimum over covers of the union and then letting $\epsilon \downarrow 0$ givesThis proves countable subadditivity of Lebesgue outer measure directly from interval covers, without assuming that any $A_n$ is measurable.