1/3 of a wall takes 1/4 hour to paint, by hypothesis.
So you still have 2/3 of that wall.
2/3 = 1/3 + 1/3
So T(2/3) = 1/4 + 1/4 = 2/4.
Which means that the 2/3 of the wall left will take 2/4 hours.
T(1/3) + T(2/3) = T(3/3) = 1/4 + 2/4 = 3/4
So to paint one wall, it will take you 3/4 hours.
Hope this helps! :)
