I dont know to put for the boxes. When I work the math out I get 5.4 more gallons but I'm confused on what to put for the other boxes

Step-by-step explanation:
One possible solution would be p being the total gallons of paint needed to buy, 250 in the first box (because 250 square feet per gallon), 250 in the second box (because you have 1 gallon = 250 sq. feet leftover) = 1600 (total)