Answer:
[tex]\frac{13+x}{2}[/tex]
Step-by-step explanation:
"A number" is the part of the question you don't know, and therefore is the variable. Let's call it [tex]x[/tex] from now on.
So, when you add a number to 13 it means you are increasing 13 by that number, or [tex]13+x[/tex].
To divide it all by 2, simply put the above expression on a fraction with a denominator of 2:
[tex]\frac{13+x}{2}[/tex]
And there is your answer.
