You can do this with a proportion. It takes 5/6 of a gallon to paint 1 wall and you want to know how much of a wall can you paint with 1/2 gallon. So set up the proportion:
5/6 / 1 = 1/2 / x
cross multiplying gets you 5/6x = 1/2
multiply by the reciprocal of 5/6 to isolate x - which is 6/5. 1/2 times 6/5 = 3/5
