For this case we propose the following variables:
x: Let the variable that represents the full-page ads
y: Let the variable that represents the half-page ads
a: Let the variable that represents the number of articles
If the magazine consists of 120 pages in total we have:
[tex]x + y + a = 120[/tex]
According to the statement we have:
[tex]x = 28\\y = 12[/tex]
We clear the value of the variable a:
[tex]a = 120-x-y\\a = 120-28-12\\a = 80[/tex]
Thus, 80 pages of the magazine are articles.
Answer:
[tex]a = 120-x-y\\a = 80[/tex]