so, his advertising expenditure is 600+ 650+700+400, namely 2350
he spent 700 on the yellow pages, how much is 700 of 2350 in percentage?
well, if we take 2350 to be 100%
then [tex]\bf \begin{array}{ccllll}
amount&\%\\
\textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\ 2350&100\\
700&x
\end{array}\implies \cfrac{2350}{7000}=\cfrac{100}{x}[/tex]
solve for "x"