#### Answer

(a) 20 sheets
(b) $256

#### Work Step by Step

(a) We can find the total area of the four walls:
$A_w = 2\times (8~ft)(12~ft)+2\times (8~ft)(16~ft)$
$A_w = 448~ft^2$
We can find the area of the ceiling:
$A_c = (12~ft)(16~ft)$
$A_c = 192~ft^2$
The total area is $448~ft^2+192~ft^2$ which is $640~ft^2$
We can find the required number of sheets of drywall:
$\frac{640~ft^2}{(4~ft)(8~ft)} = 20~sheets$
(b) We can find the cost:
$cost = (\$0.40/ft^2)(640~ft^2) = \$256$