Find the rate at which you work, that is, the # of walls painted per hour:
13 1/3 walls 40/3
------------------ = ---------- wall per hour
14 1/4 hr 57/4
Keep 40/3 as it is; invert the divisor (57/4) and then multiply the result by 40/3:
40 4 160 wall
------ * ------ = --------------- = approx. 0.94 wall in one hour, or 160/171 wall / hr
3 57 171 hr
