well, 12 issues end up as 960 pages total
so... if in 960 pages, 384 are ads, what about 80 pages?
well [tex]\bf \begin{array}{ccllll}
pages&ads\\
\textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\
960&384\\
80&x
\end{array}\implies \cfrac{960}{80}=\cfrac{384}{x}[/tex]
solve for "x"