I don't care what the area of the whole wall is, because we don't need it to solve this problem. It's just there to confuse us.
==> If he used 3/4 of a gallon to paint 1/5 of the wall, then in order to paint the whole wall, he'll have to use 3/4 of a gallon FIVE TIMES.
(5) x (3/4 gallon) = (15/4 gallon) = 3-3/4 gallons
Solving this problem may be painful, but it's not difficult.