#### Answer

32 gallons

#### 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 $ft^{2}$
One gallon needed to paint 400 $ft^{2}$
So, 12,800 $ft^{2} \div$ 400 $ft^{2}$ = 32 gallons of paint will be needed