I will ignore all the jibberish about paint colour names and simply use colour A for the large wall and colour B for the other walls
area of large wall = 12(7.5) - 5(6) = 60 square feet
area of other two walls = (9)(7.5) + 10(7.5) = 142.5 square feet
he will need 60/240 or .25 gallon of colour A for the large wall, which would be 1 quart
he will need 142.5/400 or .35625 gallons of colour B, so he needs 2 quarts of colour B
It seems silly to ask for accuracy to the nearest hundredth in the number of gallons needed.
Can you imagine walking into a paint store and asking for .356 of a gallon of paint?