You can easily solve this by setting up a proportion with walls on the top and paint on the bottom. First we can find the decimal equivalent of each of those fractions to make things easier. 5/8 = .625 and 4/5 = .8. 
. What this is asking is "if 4/5 (.8) of a wall is covered by 5/8 (.625) of a gallon of paint, how many walls (x) will 1 gallon of paint cover?" If we cross multiply we have .625x = .8 and x = 1.28 which is the same as 1 7/25 walls/1 gallon, choice C.