Let x be the mass of water added and y be the total mass of solution: 0.8(12) + 0(x) = 0.6(y) (0 is the concentration because we will add water only, with no alcohol present) --> 0.8(12) = 0.6(y) --> y = 16 L. To get x: x = 16 L - 12 L = 4 L of water
