I bought 1/2 gallon of paint then I found 1/3 of paint in my garage how much more paint do I need to have 3 gallons of paint

1/2 + 1/3 = (3*1) + (2*1) / (2*3) = (3+2) / 6 = 5/6

tinha 5/6 de litro

para 1 litro faltam 4/6 de litro

para 3 litros faltam 2 litros e 4/6 de litro ou, simplificando:

4/6 = 2/3, logo, faltam 2 litros e 2/3 de litro.