I think that this is right, but I assumed that the ceiling has the same dimensions as the wall you would need 4 gallons for 2 coats. Work: 20x8= 160 (square feet for 1 wall) 160x5=800 (i assumed there are 4 walls and 1 ceiling) 800x2=1600 (you need 2 coats) 1600/400=4 (each gallon includes 400 square feet)
The expression would be $0.20 x f. Once you figure out what the 'f' variable stands for, you have to multiply it by $0.20, and you will have your answer.
