If B is thee answer, then the amount of white in the resulting mix will be 1/2.5, and the black will be 1.5/2.5. The ratio between white and black should be 3:5 as per the problem. But (1/2.5): (1.5/2.5) is not 3:5, its 2:3. So, then B is not the right answer, right?

I am thinking E is the answer, as the smallest quantity of white that can be bought is (3*.5)G = 1.5G of white. And smallest quantity of black that can be bought is (5*0.5) = 2.5G of black. Total: 4G of paint – is that right?