Using Mathematical Conventions
Want some basic practice with variables first? Introduction to Variables
Recall:
$\mathbb{R}\,$ is the set of real numbers: $\mathbb{R} = (-\infty,\infty)$
$\mathbb{Z}\,$ is the set of integers: $\mathbb{Z} = \{\ldots,-3,-2,-1,0,1,2,3,\ldots\}$
Numbers are usually represented by lowercase letters, like $\,a\,$, $\,n\,$, or $\,x\,$.
Sets are usually represented by uppercase letters, like $\,A\,$, $\,B\,$, or $\,S\,$.
A variable with universal set $\mathbb{R}\,$ (or, any interval of real numbers) is most likely to be named with a lowercase letter from the end of the alphabet; particularly $\,t\,$, $\,x\,$, or $\,y\,$.
A variable with universal set $\mathbb{Z}\,$ (or, any subset of the integers) is most likely to be named with a lowercase letter near the middle of the alphabet; particularly $\,i\,$, $\,j\,$, $\,k\,$, $\,m\,$, or $\,n\,$.
Examples
Choices: $\,x\,$, $\,j\,$, or $\,S$
Choices: $\,B\,$, $\,k\,$, or $\,t$
(You may want to review interval notation.)
Choices: $\,t\,$, $\,A\,$, or $\,m$
Choices: $\,k\,$, $\,S\,$, or $\,y$
(You may want to review list notation.)