Answer:
0.4 gallons
Step-by-step explanation:
Length of a bedroom= 20 feet
Breadth of a bedroom= 15 feet
Height of a bedroom= 8 feet
Area of walls of a bedroom= 2(l+b)×h
Area of walls of a bedroom=2(20+15)×8 
Area of walls of a bedroom= 2× 35×8 
Area of walls of a bedroom=560 
Amount of paint in 1 gallon = 400 
Number of gallons required= 
Number of gallons required= 1.4 gallons
Hence, 0.4 extra gallons required