The thickness of the fresh paint on the wall can be calculated by dividing the volume of the paint and the area it covers. We are given the volume of one gallon which is equal to 3.78x10^-3 m^3 and an area that was covered of 20.0 m^2.
thickness = 3.78x10^-3 m^3 / 20.0 m^2
thickness = 1.89 x 10^-4 m