Answer:
First we know that the room is a rectangular prism with measures:
width = W = 15ft
length = L = 23ft
height = H = 8ft
We want to paint the room (i suppose that we paint the four walls and the roof)
The area of each two of the walls the width times the height:
A = (15ft)*8ft = 120ft^2
And we have two of these walls, then the total area is:
area = 2*120ft^2 = 240ft^2
The area of each one of the other two walls is the height times the length:
A = (23ft)*8ft = 184ft^2
And we have two of these walls, then the total area is:
A = 2*184ft^2 = 368ft^2
The area of the roof is equal to the length times the width.
A = 23ft*15ft = 276ft^2
Then the total area we need to paint is:
area = 240ft^2 + 368ft^2 + 276ft^2 = 884 ft^2
a) We know that one gallon can cover 250 ft^2
Then to cover 884 ft^2 we need:
N = (884 ft^2)/(250 ft^2) = 3.536 gallons of paint
b) Knowing that each gallon costs $28, and that we need 3.536 gallons of paint to paint the room, the total cost is:
3.54*$28 = $99.008 = $99.01
Now if for some reason you only can buy paint in whole numbers, then you can not buy exactly 3.536 gallons, then you need to buy 4 gallons, and in this case, the total cost will be 4 times $28
cost = 4*$28 = $112