A CSV column must contain only rational numbers. Which entry violates the rule?

On $U=\{1,2,\dots,12\}$, define a relation $R$ by $x\,R\,y$ iff $x-y$ is even. How many ordered pairs belong to $R$?

In a cohort of $200$ students: $|P|=120$ chose Python, $|S|=90$ chose Statistics, and $|P\cap S|=50$. How many chose exactly one of the two?

Relation $R$ on users: $uRv$ if $u$ has ever replied to a post by $v$. Which property always holds?

If $|A|=6$, then $|\mathcal{P}(A)|$ equals

If $|X|=4$ and $|Y|=7$, then $|X\times Y|$ equals

For $n\ge1$, which identity is correct?

On $\mathbb{Z}$, $x\sim y$ if $x-y$ is divisible by $3$. Then $\sim$ is

A dashboard displays $E(n)=2+4+\cdots+2n$. When $n=40$, what is $E(40)$?

In universe $U$ with $|U|=60$, if $|B|=18$ and $|C|=25$ with $|B\cap C|=7$, then $|B'\cap C'|$ equals