Answer :
a)
[tex]\bf 4walls\implies 4\left( 21\cdot 8\frac{1}{3} \right)\qquad 8\frac{1}{3}=\cfrac{8\cdot 3+1}{3}=\cfrac{25}{3} \\\\\\ 4\left( 21\cdot \cfrac{25}{3} \right)\implies 4(7\cdot 25)\implies \boxed{700}\\\\ -------------------------------\\\\ \textit{ceiling and floor}\qquad 2(21\cdot 21)\implies \boxed{882}\\\\ -------------------------------\\\\ \textit{total area to be painted}\qquad 700+882\implies 1582\ ft^2 \\\\\\ \textit{a gallon does }315\ ft^2\qquad thus\qquad \cfrac{1582}{315}\textit{ gallons needed}[/tex]
b)
how much is that if each gallon is $24.80? well, just get the product of the amount from a) and 24.80.
[tex]\bf 4walls\implies 4\left( 21\cdot 8\frac{1}{3} \right)\qquad 8\frac{1}{3}=\cfrac{8\cdot 3+1}{3}=\cfrac{25}{3} \\\\\\ 4\left( 21\cdot \cfrac{25}{3} \right)\implies 4(7\cdot 25)\implies \boxed{700}\\\\ -------------------------------\\\\ \textit{ceiling and floor}\qquad 2(21\cdot 21)\implies \boxed{882}\\\\ -------------------------------\\\\ \textit{total area to be painted}\qquad 700+882\implies 1582\ ft^2 \\\\\\ \textit{a gallon does }315\ ft^2\qquad thus\qquad \cfrac{1582}{315}\textit{ gallons needed}[/tex]
b)
how much is that if each gallon is $24.80? well, just get the product of the amount from a) and 24.80.