let's reword it, since the wording is crummy
"How many grams of active ingredient is contained in 500mL of a solution which is 30% of the active ingredient?"
namely, what is 30% of 500?
[tex]\begin{array}{|c|ll} \cline{1-1} \textit{a\% of b}\\ \cline{1-1} \\ \left( \cfrac{a}{100} \right)\cdot b \\\\ \cline{1-1} \end{array}~\hspace{5em}\stackrel{\textit{30\% of 500}}{\left( \cfrac{30}{100} \right)500}\implies 150~mL[/tex]
