3 bottom sections:
10 x 20 = 200
5x 20 = 100
10 x 20 = 200
Bottom area = 200 + 200 + 100 = 500 square feet.
Short side: 3 x 20 = 60
Deep end wall : 6 x 20 = 120
Side wall : (24 x6) - (10x3) - (1/2 x 4x3) = 144-30-6 = 108
2 side walls: 108 x 2 = 216
Total area : 500 + 216 + 60 + 120 = 896 square feet
Divide total area by area of 1 gallon:
896/400 = 2.24 gallons
It will take 2.24 gallons of paint