I found the following problem from an algebraic geometry course hold in 2003.
Let $(X,\mathscr A)$ locally ringed space and $f\in \Gamma(X,\mathscr A)$. Prove that $$X_f=\{x\in X|f(x)\ne 0\}$$ is an open subset of $X$.
What does the $\Gamma$ means in this context, or is is some course-specific notation?