#### Answer

400 pieces

#### Work Step by Step

Dimensions of ceiling are 12ft and 16ft
Area of the ceiling = l x b
= 12 x 16 = 192 $ft^{2}$
Dimension of left/ right wall are 8ft and 12ft
Area of the left/right wall = 2x8x12 = 192 $ft^{2}$
Dimension of front/back wall are 8ft and 16ft
Area of the front/back wall = 2x8x16 = 256 $ft^{2}$
Total area = 192 + 192 + 256 = 640 $ft^{2}$
Twenty rooms will be 20 x 640 $ft^{2}$ = 12,800 $ft^{2}$
Area of sheet of drywall is 4ft to 8ft = 32 $ft^{2}$
So, 12,800 ft2÷ 32 ft2 = 400 pieces of dry wall will be needed