If $T_n = \{\frac{a}{n} \mid a \in \mathbb{Z}\}$, then what are $\bigcup_{n\in\mathbb{N}}T_n$ and $\bigcap_{n\in\mathbb{N}}T_n$? 
For each positive integer $n$, let $T_n = \{\frac{a}{n} \mid a \in \mathbb{Z}\}$.
What are $\bigcup_{n \in \mathbb{N}} T_n$ and $\bigcap_{n \in \mathbb{N}} T_n$?

I'm pretty sure the first one is just $\mathbb{Q}$, the rationals. Since the set will have all possible numerators over all possible denominators. The second I'm not so sure of. It's certainly not empty, since $1/2 = 2/4$ so $T_2$ and $T_4$ have non-empty intersection. I am currently leaning towards this set also being $\mathbb{Q}$, since we will get every fraction here as well: if $a/b$ is in $T_b$, then it is also in $T_{2b}$ as $2a/2b$.

You are correct that $\bigcup_{n\in\mathbb{N}}T_n=\mathbb{Q}$. That's simply because any $q\in\mathbb{Q}$ is of the form $q=\frac{a}{b}$ for some $a\in\mathbb{Z}$ and $b\in\mathbb{N}$ and so $q\in T_b$.
You are wrong that $\bigcap_{n\in\mathbb{N}}T_n=\mathbb{Q}$. This has no chance of happening since $\bigcap_{n\in\mathbb{N}}T_n\subseteq T_m$ for any $m$ and each $T_m$ is a proper subset of $\mathbb{Q}$. 
So first note that if $n\in\mathbb{Z}$ then $n\in T_b$ for any $b\in\mathbb{N}$. That's because $n=\frac{bn}{b}$ regardless of $b$. Meaning $\mathbb{Z}\subseteq T_b$ for any $b\in\mathbb{N}$ and so $\mathbb{Z}\subseteq\bigcap_{n\in\mathbb{N}}T_n$.
We will show that $\bigcap_{n\in\mathbb{N}}T_n\subseteq\mathbb{Z}$. So assume that $q\in\mathbb{Q}\backslash\mathbb{Z}$, i.e. $q=\frac{a}{b}$ for some $a\in\mathbb{Z}\backslash\{0\}$ and $b>1$ relatively prime. Obviously $q\in T_b$. Take a prime number $p$ not dividing $b$. It is enough to show that $q\not\in T_p$. Indeed, $\frac{a}{b}=\frac{x}{p}$ implies $ap=xb$ which cannot hold since $b>1$ is relatively prime with $a$ and with $p$.
This shows that $\bigcap_{n\in\mathbb{N}}T_n =\mathbb{Z}$.
