Union of set and interval I'm working on finding the boundaries of sets, I feel like I understand this. However, one problem asks for the boundary of $\{1,2,3\}\cup(2,4)$ and I'm unsure as to how to take the union of an interval and a set. 
Here's my thoughts:
The union will include points $1$ and $2$ and then the interval from $2$ to $4$ but not including $4$. I'm not sure how I'd write that though. Possibly $\{1,[2,4)\}$?

You get the union of a bunch of points and the open set $(2,4)$, as you note, but I would write this as $\{1\}\cup[2,4)$.
