[tex]\bf \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{65\% of 20}}{\left( \cfrac{65}{100}\right)20}\implies 13[/tex]
so, 13 of the 20 tables have 6 chairs each.
that means, the remaining 7 tables, have 4 chairs each.
13 * 6 = 78 chairs total for those 13 tables.
7 * 4 = 28 chairs total for the remaining 7 tables.
78 + 28, that's how many.
